After buying various grade generic memory foams on ebay to save a
dime, I highly recommend biting the bullet and getting the real stuff
from an actual glider/aviation supplier... *it's not the same stuff.

Yep, I have a 1x16x18'' piece of the "real stuff" on order. But, what
do you think about using much cheaper firm foam underneath?

I will let you know as soon as I figure out the shape of the plywood
base.

Thanks!
Bart