There is a set of 11 plastics on eBay that includes the slings. Fairly yellowed and the price is ludicrous at $114, but if you 'had' to have them. It took me over a year to find mine. Good luck in your search.

Mine had broken corners, I think bottom corner on each. Someone had cut corners off another one maybe, but the corner pieces have the matching black star pattern and overlap over the broken corner. It isn't the best fix but if it's the same corners I can mail you them if I can find them, I think I still have them somewhere. It will be way better than looking at that missing corner. Or you could get plastic corner protector washers, paint them black and put them over the broken corner.