So you have a virgin bumper, congrats! You want to/have to mount a license plate, but don't want to screw or drill your bumper. Here is a short tutorial that will show you how to do it quickly, cheaply and effectively.
What you need:
Phillips Head Screwdriver
1 Oem Lexus Front License Plate mounting bracket (comes w/2 screws to mount plate to bracket) (dont mind mine in the pic, I cut it so I can mount it upside down for my plate higher to clear my front lip)
2 4" long 1/4" bolts
2 1/4" nuts for above bolts
2 ~3" washers
Step 1. Remove grill. (no pics, do a search)
Step 2. Center/Place bracket where you would like, on my car, I centered it. Thread 4 inch bolts into holes and place washers on the other side of the honeycomb grill. Fasten with nuts and tighten.
Like I stated earlier, mine is mounted upside down so my plate clears my front lip, you would most likely mount yours right side up and up close to the top of the honeycomb grill, so when your plate is mounted there is no visible gap between the bumper and the plate.
Step 3. Attach License plate to mounting bracket via 2 screws.
Step 4. Reinstall grill.
*you can probably use a shorter bolt.
for those who have pm'ed me, emailed me and im'ed me to show you how to do this, i apologize for being a lazy bum and taking so long to write this.