Checkout these super cool and easy to install axle mounted license plate holder! Just mount it behind your axle bolt head/nut depending on which side you want it, position the plate at the proper angle and lock it down. Viola! Bob's your uncle!
The predrilled hole is big enough to fit the 20MM axle which is mostly standard on our beloved twins and fours. Also, the 1/8" thick laser cut TIG welded construction makes sure it won't vibrate when hitting the TON.
Rock n' roll, bubba!