need one on the leading edge of you car, i.e bumper, and can get done for not having it there, even if its in window ( unlikely but you might get a bobbie in a bad mood )
could do what my mate did, get a few velcro sticky pads, put a few on the back of your bumper then a few on the bumper and you can stick it on then when it comes to shows ect the sticky pads on the bumper can be taken off.
or if you have a r32 bumper you could proberbly knock up some numberplate holder that can be wrap tied/ attached to the grills. (not sure if you could do this to the gti, not sure if the grills ect are big enough.