I repair my GWS planes with yellow carpenter's glue and clear packing tape.

If there's a missing section of foam, I mix 50-50 yellow glue and polyurethane glue, using tape as a backing (for one smooth surface.. on top) fill the gap with the foaming mix. When that dries, carve the underside reasonably smooth. don't bother peeling the tape (unless the plane it getting too heavy...)