I get mine from flexi plates through eBay. It's a legal size and has a bs number on the plate. Not sure of exact legality but the mot guy seems happy enough
I've got flexi from ebay with 2 holes reverse drilled (drill in reverse melts it's way through and leaves a non-cracking edge) then bang in some dress making collets to give a rounded edge to the holes, and tie it on with yellow elastic cord from ebay. The flexi plates can be broken if bolted on but not with this! The cord does wear, I tie a thumb not in each end under the fender and when it starts to wear I just move the knots along slightly, every 4-5 ride outs or so. Not broken the full legal size and BS marked plate since started using it early last year.