I finished building my ball differential. It works well in a drill press, but I don’t yet have a car to test it in. You will need to order a couple bearings, but I found them cheap on ebay if you order packs of 5 or 10. Everything else is common parts. It would be nice if someone made one and tried it out.
https://www.thingiverse.com/thing:2809042
If this doesn’t work, I’ll probably try a planetary differential.