Take a piece of steel strip, cut to length, drill 2 holes correct distance apart for the license plate, take another piece, drill holes for whatever it's being bolted to (not visible in photo), weld it to the first in the right spot, grind flush, bend it to the right angle, paint, done.
Can't weld? make a cardboard template to get the shape right, take a piece of steel sheet, transfer your pattern onto it, cut it to the right shape, drill holes, bend, paint, done.
If you have a buddy or know a shop with a CNC plasma or laser cutter, it's even easier.
Lots of ways to get it done, depending on what technologies you have at your disposal.