More updates to the previous 3D printed prototypes. The pieces are available on Thingiverse and Shapeways. These pieces were 3D printed by Shapeways in their White, Strong, Flexible material. Screwing the set screws directly into the plastic instead of tapping the holes kept the piece viable for several months before it wore out.
Managed to get a pretty decent locating fit for a 3/8" x 5/8" x 5/32" sealed, pre-lubricated ball bearing from VXB. They have a lot of options in this size. I ordered two to choose from, but only one fit into the printed flange piece easily with just my hand pressure even though they were rated for the same maximum OD and tolerance. So I think the current design is close to the edge of hand pressed fitting for the low tolerance of the current SLS 3D printing process.
If you look carefully at the bottom edge of the flange where it contacts the drill adapter, you can see there is a step-wise pattern; this is because I tried to give the surface that contacts the drill adapter a gentle slope to match the slope on the front surface of the drill adapter. It basically works, but the 3D printing leaves this step pattern instead of a smooth slope. Probably not worth the trouble.
Here's the whole thing assembled. Note that the square driver piece is taking on a healthy coffee color due to the plastic material being porous and absorbing bits of coffee grounds. I've had this piece a while now and it's showing no signs of failing, so I think the 3D printed parts are reasonably successful for medium use. It's also kind of nice to know that the plastic part will fail before you kill the KitchenAid motor if you happen to jam things up some how.
Here's a picture of the extra 5/8" OD bearing I bought that didn't quite fit nicely. I also bought a 9/16" OD needle bearing to try, but it seemed better to stick to something with less depth and the 5/8" OD ball bearing was a small enough OD to work.