Polyester foam, basically the foam used for seat padding at any fabric area in a store or a fabric store, wrap it in like some pleather or black cloth for a professional look!
Much like zachturner009 said. I use high density foam for chairs and such. I think it's like 1 inch thick but I use scissor shears to cut the foam clean to size I need then I wrap it in a rubber cloth material that has honeycombs on it for a cool look. Best part is loctite super glue bonds the rubber material together very well and therefore no stitching needed. Looks super clean and professional.
I got everything from jo Anne faberics. The hi density upholstery foam has a green color to it. I think I got a 1 inch thick sheet. The black honey comb material was found over in the fabric section with all the cosplay stuff. I just wrap it and tack in place with the loctite 5 sec super glue, then I velcro all my padding in so it's removeable.
Honestly, I made my CQB Helmet so large i just went out and bought MMA headgear and it popped right in perfectly snug. Also made it incredible easy to wash and keep the funk away.