John,
I don't think I told you when I was there that I got the plywood for my
boat at Windsor Plywood in Great Falls. I could only get 4mm. They
called it marine grade and it came from Brazil. It looks like mahogany
and has three equal plys. The quality is so, so. After I skinned my
boat, I found a small area of loose plys. I was able to cut it out and
fill.

I would call them and see if they can get anything else. They had a
few sheets left when I picked mine up. I'm not sure if 4mm is what you
want for wings?

Dale