Im not sure of the compound thats under there. But after I carved the model out of clay. A company in PA had this foam mixture, That you can decide the density of the foam. 1st part was to submerge the face or seat surface part of clay into it. When that set up. It formed a rubber female mold. Then you would pour that other mixture in there. When that set up. U had the foam part of the seat that the upolster covers with the vinyl or leather covering. The foam is exactly what ya find under any mc seat.
When I get a chance I'll look up the company.