After a trip to the metal shop I finally conquered the shifter side of my rear set foot controls on the Slingshot. So step 1: Get a good sized piece of 1/2" thick aluminum.
Step 2: Using all the knowledge & skill you've obtained from watching every episode of Biker Build-Off 27 times, make a pattern and get your plate bolted to the bike.
Step 3: Drill a mounting hole for the rear set and make sure it lines up with the right side. Trim the excess material, and try to not cut any fingers off.
Lastly, send your roughed out mounting bracket out to be finish shaped and polished. Real big time builders don't do that shit themselves, that's for the little people to do.
I made the tank mounts and got the tank hard mounted on the bike as well, but I've put enough pictures of that up already.