I'm not sure if anyone makes them, I've not seen anyone I should say. I have however, seen a guy use some ragged out old tail lights to make a thin fiberglass mold from and then fit them in and finish. I know you said you were interested in weight, but I couldn't see how a thin fiberglass "filler panel" would be any more weight than a sheet metal one. Not to mention, even if it were sheet metal, the weight savings would be miniscule IMO.