In the UK we can get a hard foam that is used for orthopaedic padding and seating. I think it is known as M60 foam and can be bought from good upholstering suppliers. it can be filed or shaped with a sureform.I use a 1/2 to 3/4 re constituted foam that is a bit firmer then a cover of M60 then shape to requirements. If you want more help send a PM and I will give you my number.
Pete Smallwood, Irlam M44