Thanks for the pictures of your Wanderer dinghy Martin.
Mid way between the Enterprise and Wayfarer, I've always felt that the Wanderer is an under-rated design, and it's nice to see that you're getting fun out of it.
A stretchy nylon material should work well with the barrel cut and you seem to have the main panels well organised.
Looking at the photos there are two thoughts which come to me. In an ideal world, with and infinity thin mast I would make the slot width zero. As the real mast gets thicker I feel the slot should get wider, to let the air flow past the mast, so I suppose I think of the slot width to be proportional to the mast diameter. Thin mast narrow slot, fat mast and wide slot. You have a fairly thin mast but to my eyes the slot looks very wide, which may (or may not) lose some of the benefit of accelerated air flow. (If I sound rather vague here it's because I'm not sure but it is a thought).
The other thought is that as you get the rig settled down it is more important to remove all diagonal creases from the jib panels than from the mains, as it is the airflow over the cambered jibs that does the majority of the useful work. Creases in the jibs distort the camber badly.
I don't see telltales on your jibs, but they are a great help, and you'll probably find that with them streaming you'll get your 'best to windward' performance. Keeping the jibs pulling should be the key to best performance.
Congratulations on your good work. Keep it up. Please don't take my comments as a criticism, as I only want to say what I have found out in the past.
Cheers, Slieve.