Your wish is my command Paul, A proper sail is n the go as we speak.
if you were stuck on a deserted island with no water...
you had cloth a sewing machine and power...
would you put on pockets, with a grommet in the luff and one in the leech, through them you could have a small line pulling them towards a hole at the end of the batten, making it truly 2 holes, since every stick has two ends...
if I could find battens with 1 end only of course I would drill only 1 hole, yet for now that option seems a touch out of reach...
So how about the previous option, tension luff and leech with lanyards going into a hole at the end of the batten, which will surely be 5 cm longer than the sail.
5 cm enough, or more???
Depends on normal, to light stretch of my material over 4 meters. So is 5 enough??
7 seems better.
how would you do it? remember you are on a deserted island...
Keep Junking my friend