For a kayak plug I used 2 inch pink styrofoam insulation from the lumber yard, roughly shaped and laminated together with construction adhesive. This material carves easily with a wavy-edged bread knife. Be careful not to let the adhesive get too close to the surface of the plug. Fine shaping was done with lightweight, sandible drywall mud, and finished off with a coat of epoxy paint, and the usual waxing and mold release. The mold separated from the plug with no trouble. The plug was formed on a sheet of cheap, waxed paneling, so I was able to get a flange on my molds. Everything I needed I got from the local building supply.