Paint is permanent. And it's fun to paint your number plate on the bonnet.
Most coppers don't really know (or care) whether it's legal or not but you'll need a friendly garage to pass it for the MOT
if the car isn't old enough for it to have been legal when it was built.
OR... you can just whack on the standard flat plastic job on the day of the MOT of course...
1) Buy a couple of copies of the Daily Telegraph (the only broadsheet newspaper left).
2) Mask off the area you want to paint and plaster the rest of the car in newspaper (spray paint Luuuuurves to travel and a light haze on the windscreen is no fun).
3) Sand it as described in previous posts.
4) Paint the square / rectangle as desired with matt paint.
5) Leave it a few hours.
6) Second light coat of matt paint and allow to dry thoroughly.
7) Buy a vinyl square version of your number plate online and very, very carefully cut out the number to make a stencil. (this should of course be step one but you would have stopped reading at the mention of vynil....)
Or make your own with sticky letters from B&Q for sixpence.
8) Cut each number separately out of the stencil, gently heat (not in water) and apply to the square.
9) when it looks right spray the opposite colour to the background - but don't take them off ! Leave a couple of hours and spray again... Leave a couple of hours and remove the stencils.
Now you're looking good....
Had this on my Mini for years. Was never stopped, never had an MOT issue even though it wasn't technically legal for it's age.
Which gets you thinking about all those '80s and '90s MX5s.... ???