There are some really good reasons why you want to use dynafoam, or something very similar - a conformal foam in other words.
Research has shown that a soft seat cushion, in a hard landing, will result in your still travelling downwards through the seat cushion, whilst the aircraft is bouncing upwards. The result can be a shock to the spine as much as 80% greater than that if you'd been sat on a bare unpadded seat.
The chap, incidentally,who worked all this out and is something of a hero in the cleverer bits of the gliding and light aeroplane communities. He is a lifelong glider pilot and retired GP who has spent the last 20 years or so researching glider safety - because he could. He's published a lot of papers on this, mostly in Technical Soaring, which is published by OSTIV - fellow by the name of Dr. Tony Segal, who lives in Uxbridge.
But, in a nutshell, use dynafoam, and you can buy it from AFE. Do not use soft foams such as you might have in your sofa, because of the risk of injury in a heavy landing.
It is also best to ensure that you use flame retardant foam (which dynafoam is) and covering material (read the label!).
G