Okay, I have a plan!
The ends of the windshield wiper motors are DD 1/2" x 1/4". What I do is I take a really thick or strong piece of metal in flat stock form, such as steel or thick aluminum. I take a 1/4" drillbit and carefully drill two holes directly next to each other. I file away the sharp edges and the result is a DD bore the right size I need it! I drill a couple holes of the ends of the flat stock steel or aluminum, and I've got a hub I can mount to my wheels!
What do you guys think?