Unfortunately its not just a lens cover you can screw on but, a complete unit, which is easy to replace from under the vehicle or by removing part of the rear wheel well cover.
https://www.ebay.co.uk/itm/175862628037?hash=item28f23abec5:g:T3QAAOSwjgBkIXIV
Do take into consideration, if you go through insurance the excess will be £100.
I would perhaps ring rounds some of the breakers yards and see what prices they quote you for the part.