Abstract: Method of making a mold for vacuum thermoforming is made by (a) applying a polyester gel coat to a master pattern; (b) applying a rigidizing mixture of a thermosetting resin and glass to the gel coat; (c) constructing and bonding an egg crate framework to the cured thermoset resin and glass fiber mixture; (d) filling the spacings of the egg crate framework with a polymeric rigidizing foam; (e) forming a vacuum plenum on the foam-filled egg crate; (f) attaching to the vacuum plenum a means for introducing a vacuum and; (g) forming air passageways through the gel coat which communicate with the vacuum plenum.