I'm a short guy. I've used seat cushions in several airplanes, though usually behind me, rather than under me. In one airplane, I had to unbolt the seat tracks and move them forward one bolt hole (A PB4Y-2, which is a version of the B-24), and use a cushion behind my back. I needed a lot of rudder, full throw, and couldn't get it without getting farther forward on the pedals.

At the time, and in other aircraft subsequent, fires were always a concern; I was also required to wear nomex, gloves, overlapping leather boots, helmet, etc. The seat wasn't really an issue, however, and the cushions never raised any concerns. The cushions were bought at the local department store, and were dense pads used for outdoor seats. I sewed several together to get the right thickness and spacing.

If you absolutely need a something which doesn't sustain flame, you might try an upholstery shop that does work for aircraft interiors. You can get fabric that's already been flame-tested, and a foam that doesn't sustain a burn.

If you're wearing flammable clothing, then your seat cushion isn't really the concern you might think it is; forget the cushion; you're already a candle.

Very sensible to get dynafoam to sit on. The foam behind you isn't so critical - the major point is that it must be more-or-less incompressible so you don't have a "feet off rudder pedals" experience.

You don't actually need a cover for it - so for a week or so you can just carve it and sit on it. (However, it is a bit brittle and over time bits will rub off and it will get dirty)

Covers are easy to make if you aren't worried about appearance. Get a bit of suitable cloth, wrap the dynafoam as if it was a present, stick the cloth to itself using "wonderweb" nylon webbing that melts under a hot iron, and do a few quick-and-dirty tack stitches along the edges.

Vacuum Bead Cushions
 

You might like to look at a type of booster seat which is a bag of beads from which you evacuate the air to keep it in shape.

Mechta Senior worked on these at RAE Farnborough in the late 1960s, for, if I recall correctly, 'V' bomber applications. The objective was a conformal cushion which would be rigid enough to be used on an ejection seat.

A quick google showed the current application of these is for wheelchairs.

http://dlfdata2010.dlf.org.uk/produc...4&groupid=1140

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I would endorse everyone else's comments about using a layer of Dynafoam as well. Just one thing with the stuff is that you really want it in an outer covering as it goes crumbly quite quickly, maybe due to ultra violet light exposure.