Welcome to the club I just put a giant fender washer under the thumbscrew. It was wide enough to land on the remaining plastic. Holding ... so far...
Z.
I'll have to try that at some point, the one on our bx2660 has both sides broken.Make a temp plate out of card board of the good side, then cut a thin sheet of aluminum the same as your temp plate. Use small pop rivets and pop over what is left leaving room for the thumbscrew. Might want to have the head of pop rivet on the under side in spots.