When I swapped the factory front bumper on my Unlimited for an aftermarket one, I was left to find a place to mount the license plate.
I didn't want to drill holes in the new bumper, so I devised a very simple mount made of 1/4" threaded rod and 1/4" I.D. fuel line.
I cut two pieces of threaded rod to about the right length to wrap around three sides of the bumper, then slipped the fuel line over it. Last, I bent the threaded rod at the corners, forming two rectangular U bolts with a nice rubber sheathing to protect the bumper. The ends of the rod go through the original mounting holes in the license plate, and are capped off with locking nuts.
Simple, cheap, and easy.