If I was in your predicament, I'd just make one out of plexiglass sheet and some suction cups that you can buy as replacements for radar detector brackets. Shouldn't be that hard to cut out and assemble.
Another option (and lighter) would be to surround the sharp edges of the license plate with some foam rubber (something like a door edge guard, but softer). Glue the foam/protective edge to the license plate, then simply use the bolt holes in the plate to mount four of the suction cups. I doubt it would go anywhere, and if it did, the foam rubber would minimize/eliminate any gouges to the dash work. The foam rubber would also act as an insulating barrier and keep wind from seeping around the wind screen and lifting the plate away from it.
Good luck!
-Dean