t'interweb is your friend, search for "Kennzeichenhalter rahmenlos" (frameless numberplate holder). I've got these on my Volvo:
https://www.amazon.de/-/en/GENMAG-Fr...=sr_1_5?sr=8-5
You end up with what looks like four black tabs holding the plates. Had them on my car for 8+ years. Put some extra glue on the tabs and no one is getting them off without destroying the plate.