square out the missing spot, epoxy in a new piece of signfoam, recarve & repaint.
A "Dutchman" patch. Cut the piece for the patch, "draw" around it with a sharp knife with it over the damaged part then route out the recess and glue in your patch. Sand flush and prime/paint. This way your patch is also your pattern.square out the missing spot, epoxy in a new piece of signfoam, recarve & repaint.