Check out the book making a kevlar canoe. It documents how to make a foam plug. might be of some help.

my $.02

: 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.