I have previously asked a question about plywood for my hatch rims. http://www.kayakforum.com/cgi-bin/Building/index.cgi/page/1/md/read/id/229506/sbj/strip-plywood-for-hatch-rims/
I have finally found some 4 mm plywood. 3 mm (about 1/8") plywood seems impossible to find i Denmark where I live. We don't have "door skins". Doors is usually bought premade.
So I will make my hatch rims from this 4 mm plywood.

I have found Rod Taits/Orca boats guide to make plywood hatch rims. http://orcaboats.files.wordpress.com/2013/10/plywood-hatch-stops.pdf. Rod uses 2 layers of 1/8" plywood to make the spacer that will make room for the gasket. That gives a total spacer thickness of 1/4" or 6 mm.

Now I can't decide if should make my spacer 1 (4 mm) or 2 (8 mm) layers thick. If I choose 2 layers i guess I will to find a gasket about 16 mm thick. That seems like a lot to me.

Palle Dam, Denmark