We used Nylex cab foam or lead impregnated vinyl for marine engine covers for years but don't think it is still available. Here is a link to some currently available stuff.
http://www.spec-net.com.au/pyrotek/sylomer.htm
I can't vouch for it but looks very similar.
Of course the old fashioned cheap way out was to use 100% woolen carpet glued to the inside of engine enclosures. Just be prepared to throw it out every couple of years if it gets oil soaked.
Cheers,
Graeme