I would get some 1/4" thick mdf, trace the outline of the counter and cut it a 1/4" back or so, attach with construction adhesive (PL400 of PL Premium) and a few short nails, then paint the mdf. You could even give the edge a little round over ( a router works great for this, but you could probably hand sand it if you don't have one. How noticeable is it from eye level? It looks like they folded the fiberglass cloth over on the corners. Kind of ugly.