i done this, used bit of mesh and stuck some cardboard on the back with filler. I then made a new light with the orginal boot handle and a LED strip. Chek my blog for pics of how it looks. Called the micrat.
Wel technicaly u dont need a number plate light as long as u dont drive at night, u can get whats caled a daytime MOT,they dont check any lights but then can only drive during the day. Silly but posible.