Hi Graeme, thanks for the info!
Luckily I already have a well-functioning junk sail for my folding boat.
After the sailing season ends (i.e. - not before October) I will try to focus on solving the last two conditions you quoted - durability and short preparation time.
I have never been a DIY enthusiast, but now as a retiree I have found that DIY is a very inspiring activity. I have learned a lot of interesting techniques and I am trying to learn some more.
As for my boat - you have to remember that it is not a pontoon. It is a skeleton construction. I suggest watching the film (from five years ago, i.e. before the invasion), in which testers (from Russia or Ukraine, I am not sure) - test the original version (i.e. rowing). You can choose automatic translation of the subtitles into any language.
Pay attention to minute 9:52, when the young woman lifts this boat up. Of course, my sailing version is heavier, but the hull is much stiffer than the pontoon and very light. Unfortunately, it also has its weaknesses - it is quite delicate and preparation for sailing takes a long time.
Interesting facts - Another foldable sailboat (Mewa 2, Mewa 3) was produced in Poland (before Perestroika) and was popular in the former USSR. But that's history.