In my younger days I used to run with a set of french style pressed plates on a Megane cabriolet (car was ok 2.0 IDE, but the plates looing back were terrible). They were stuck on with sticky back velcro dots so that I could swap them out easily come MOT time etc.
I recently bought some VERY stick velcro dots from Wilkos (not for number plates, but would do the job very well)
EDIT: can just make out the plate in this picture. No, i'm not female, or gay. No idea what I was thinking.
I was thinking of replacing my standard plates with ones fixed to the bumper on velcro tabs meaning i can easily swap the whole plate when at shows / track...