I used a method I read
about in the EAA builders
resource articles.
This method had you take
2 layers of 3/16 foam core
and epoxie them together to
make a 3/8 thick board. Then
cover that with a layer of
fiber glass.
I had made my seat backs that
way, but the first time I
climbed out of the cockpit.
I put my weight on the top
of the back. The fabricated
board buckled.
Removing the foam was a pain!
I had to cut it off the back
with my bread knife.
I replaced the foam core
fiber glass back with 1/8
plywood. It is stronger
and lighter.