In reply to Kevin: There is also the Welsford Pathfinder dinghy Cirrus which is featured as Boat of the Month here (scroll down to March 2023) which features a SJR similar to (but not identical to) the Amiina rig.
Jan might note that it is a 4-panel sail. It also has a proportionally larger upper panel. However, I would suggest to Jan, for a first sail, to follow exactly the proportions of the Amiina sail, leave off the bottom panel, and don't bother with the split in the top panel. That is, if he finally settles on a SJR.
Jan: I really liked the idea of extending the swinging boards... With a new sail it may not be necessary. The current boards may be fine how they are - or (as in standard dinghy practice) it may be sufficient to adjust the angle of the swinging boards, with the lifting tackle. I would suggest leave that question for now... don't change too many things at once.
The sail catcher can be convenient, also when transporting by car, because the entire sail with battens is packed like in a case and it is easier to keep order. But doesn't such a large surface cause too much wind resistance? Certainly I find mine convenient to keep everything packed together for transport. You can use "hook and loop" tape at the top. to enable the sail catcher to become a closed bag, ie a sail cover, if you want. I have no idea if it "causes wind resistance", I would suggest that if so, it is a minor detail. In fact it is tidy and in line with the sail and may well disturb the airflow less than the exposed panels of a conventionally reefed junk sail. (I look at it as a lower extension of the sail).
Spanned downhaul parrels… So a separate downhaul must be used for each panel? No, they are spanned, so that each one operates on two panels. You can refer to Slieve's notes. The relevant chapter is here.
On the 5-panel Amiina sail, two of them is all you need. The only proviso with the spans is that you need enough space between the boom and the "deck" for when the span is in the reefed position. Slightly more than half the width of a panel is sufficient. I believe this need not be a problem with your set-up, but take care not to have the boom too low, as you need to allow enough room for the spans to fully extend.
Also, provided you do not alter the proportions of the Amiina sail, you will find that no other control lines are necessary.
If you use a 4-panel version of the Amiina sail you might be able to have a standing "downhaul" and short standing batten parrel on the boom (ie fixed, not needing any adjustment) as you would anyway, and just a single spanned parrel downhaul for the two battens above the boom. Perhaps the top batten and the yard could each just have a standing short batten parrel. That would give you the ability to reef one panel, or two panels, and would reduce everything to just one control line. I haven't tried it though. I find two control lines not too difficult to manage. They are left free when you hoist and just need a tweak when hoisted, tweaked again if you reef. A couple of cam cleats is all you need. (If you change the shape of the sail or the "planform" then you may need additional parrels or hauling lines, you will have to find that out for yourself. That is why I would stress, stick to the exact proportions of the Amiina sail.)
I would like to limit the number of lines as much as possible, as it can cause lines to become tangled in the trunk of the car and when preparing the boat for sailing. It's always a problem if you have to dismantle everything. I have my halyard, lifts and mainsheet block attached by quick release clips. (The halyard is unclipped from the yard and remains with the mast. The mainsheet block complete with mainsheet is laid neatly in the sail catcher, together with the sheetlets, nothing is un-roven). The parrel downhaul spans are also just clipped on to the actual downhaul ropes, so the parrels stay clipped onto the battens and go into the sail catcher, while the downhaul ropes ("control lines") stay with the boat. So the entire bundle (sitting tidily in the sail catcher complete with its downhaul spans, battens, sheetlets and mainsheet) are all stowed in the sail catcher free of tangles and ready to be re-deployed. The sail catcher is now a sail bag, with its bundle inside, with everything unclipped, and thus separated from the mast. For transport, it can be stowed in the boat or in the back of a car, without the risk of tangles. Clipping everything back on takes a little care, but it is still only a few minutes.
...soft rubber tubes in the place where the battens will rub against the mast...Metal battens on metal mast might need fendering, others can advise you on that.
I'm also wondering how you attach the sail to the battens. For a simple little sail I have found wooden battens (split, one half each side of the sail, and screwed together through the sail) to be a quick and easy way, which does not require batten pockets or fendering against the mast. That is what you see in the photo of my boat. If you are only making a polytarp sail I would certainly recommend it. (For a better sail - well, that's a matter of opinion. I am trying batten pockets and aluminium battens this time round - already finding that it creates a great deal more work and for a little dinghy sail I doubt if batten pockets, metal battens and fenders, and hoops and loops are worth the trouble. Others might disagree. For a dinghy I would go back to split spars made from wood and screwed together through the sail - preferably some nice straight-grained spruce, though I have had to make do with pinus radiata which is not as strong and a bit heavy, but it still works. Rigging a dinghy is a slightly different proposition to rigging a larger sailboat. Split timber battens are quick and easy to assemble with a cordless drill-driver, and also easy to dismantle if you ever need to. Not everyone will agree with doing it that way.
I just want to make a general comment. We are focusing here on many minor details and forgetting the big picture (one of my many faults). I think your first priority is to produce a scale drawing showing your proposed sail plan in relation to the lateral plane of the boat (with the boards in their normal position, and the rudder.) From analysing a photograph I felt that a 4-panel Amiina sail would fit your mast and probably bring the centre of area of the rig far enough forward to solve your problem and perhaps leave a little weather helm you could tune out with the centreboard.
It looked pretty good to me, but only based on a photograph and not yet confirmed. Now, when I see your "avatar' showing your proposed sail, it looks to be much further forward (and too tall) - but without the swinging boards in the picture it is impossible to be sure if it is too far forward or not. Without a proper scale drawing, who can say? Who knows: you might be better off with a slightly lower balance sail than 33% - if you over-do it then you will have lee helm, putting you in a worse position than you were before.
So, before settling on the SJR and cutting cloth - and certainly before worrying about the minor details, make sure you have carefully considered the relationship of the centre of area of the proposed sail, in relation to the centre of area of the under-water profile of the hull (with the boards deployed in normal position) - preferably with a scale drawing. The centre of area of a Amiina sail will be near enough in the same vertical line as a vertical line through the midpoint of the boom.
The goal should not be to have a SJR - the goal should be to have a sail which has the right balance to suit your dinghy, with your fixed mast position (and current position of swinging boards).
[Edit: I scaled up your "avatar" drawing as it just doesn't look good to me. I noticed it is similar to, but different from Amiina's sail in a couple of respects.
That's up to you, although I am sure if you contact Slieve he would be helpful and willing for you to use his design. Of more concern: your proposed sail appears (by eyeball) to have much more balance than 33%. I think you definitely need to re-consider that.]