Thanks Snochaser and RCLogger. I thought of buying some rims but did not know what I would need. I also wanted something that looked like it had planetary hubs. The nice thing about having a lathe and milling machine, I can make my own rims for about $4 apiece (and a whole lot of time).
It doesn't show up well in the other photos but the hub fits nicely into a bored out space so there is a lot of metal on metal contact between the hub and rim. I also put a slight indent in the motor shaft so the set screw will be more secure to prevent the hub from both spinning and pulling off the shaft.
The hub I showed is actually the second I did. The first one took two drill bits (broken off in the hub), one centre bit (broken off in the hub), a 1/8 end mill (broken off when I accidentally went the wrong way milling out the bits) and 2 dulled 3/16 end mills (milling out the end mill). I eventually got everything out and filled the holes with JB weld ... but what a pain.
I made a small attempt to get the broken bit out of the second hub but soon gave up and filled the hole with JB weld.
Here are a few more photos:
