This might sound stupid, but have you considered 3M automotive tape? I've mounted a plate this way as long as 18 months (and it probably would've stayed on longer, I just decided to stop running front plates altogether). Never had a problem with weather or washing the car or anything. And no damage to the paint when I decided to take it off. Just needed a little Goo Gone (or equivalent)
I would just make sure that you use enough tape (all four corners, maybe even center) that the plate can't contact, and possibly scratch, the bumper. And I'd use a license plate frame so it looks nicer and not so tacked on.
Just a thought. It was a good temporary solution for me.