I used something like this:
Universal License Plate Bracket | B428-09 | Nuera Trailer Parts
I pulled the light, modified the bracket to fit/work and then remounted the entire assembly. I think I picked up the piece in the trailer supply section at Academy or Tractor Supply.
Current TV:'14 F350 Lariat CrewCab, 4x4 Short Bed SRW Diesel w/ Pullrite 2700 (16K)
Current Fiver: '15 Puma 295BHSS Anniversary Edition
Past TV: '13 F150 HD Payload SCrew EB, '08 Tundra CrewMax 5.7L, '02 'Burb 5.3L, '97 Dakota 3.9L
Past Trailers: '09 Wildwood LE 27RB, '02 Palomino Yearling RL, '87 Jayco 1006dlx