I've glued in drain plugs for the internal water tight compartments (air pressure tested). coat the inside of the female sleeve of the plug with wax, masking tape or mold release. mix up a thick batch of west with colloidal silicate in it. figure out some way to clamp it. then then coat the out side of the sleeve. use plenty. now that the sleeve is inserted to it proper place. don't laugh stick your finger in the hole and mold the excess into a flange in the inside. mold/ remove the outside "goop", let dry remove wax/tape. put the plug into the sleeve and go sailing.