Step 1: 1/4 20 - socket cap bolt and diy plate
Hot Rolled 1/2 steel - You can can get this at any bigbox or order it here (.59c for 12")
Socket cap bolts - the ones shown here are from 8020 as they were left over, however you can use any 1/4-20 socket cap screws with a washer if required. These are 1/2", yet 5/8" lengths should work.
You only need to drill and tap two holes on 1.5" on center or what ever distance fits your application.
Step 2: Socket cap and square nut - 5/16
Here are the parts. I purchased mine from Lowes but I've added links to online sites.
5/16 Socket cap @ 3/4
5/16 Lock washer
5/16 Square Nut