Has anyone found a suitable material? 
Hi Martin,
I am still looking...Went to a fabric store, but no luck.  The fabric people thought that it was
a paper type product 

I tried a plastic material, but it did not "billow" very well 

On one of my R.C. Playmo ships I used a nylon ripstop material.  I cut it with a
hot knife so it would not fray.  This worked well for actual sailing.  I may
try it on my 4444 ship......
Good luck, and happy sailing!
Jimbo