Re : Front seat bottom-layer "cushion"
Hi Bruce,
I used about 2 inches of styrofoam between the base of my RV6 seat foam and the fuselage floor. For a start it is a lot lighter than seat foam, and in the event of a very hard landing - one where the gearlegs are mashed and the fuse hits the deck hard - the styrofoam may make the difference in energy absorbtion, in the same way styrofoam is used in motorcycle hemets.
The only downside is that it is not very nice when it burns, but I figure if it gets to burning, then pretty much all else is lost anyway.
Cheers
Martin in Oz