On to the bobbin platen. I went with a light-weight plexi disc. It's just two pieces glued together with CA. To mount the disc, Lollar recommends tapping a shaft collar for three screws. Again, the Sewing Machine Anatomical Donor provided: this worm gear was tapped for three 3/8" 4-40 machine screws.
Getting the disc to run true took a little fiddling, but I got it down to about .05" or less of runout. I discovered that the plexi didn't glue up perfectly flat, so some of that wobble is in the disc itself. I think that's within an acceptable margin of error for my purposes. Heck, when I visited John Sonksen's shop, we wound a coil by double-sticking a bobbin to the turny knob of a whole, unmolested sewing machine!
And here is the final picture of construction thus far. I drew a bullseye on the disc to help with centering the bobbin since I didn't exactly get the platen centered on the shaft.
There are a couple things I plan to add before I call it complete. When Sonksen let me take a turn at winding the coil, I really had a hard time seeing the wire as it payed out and laid down in the bobbin. I painted a 6"x6" square of plywood flat black, which I will attach to the back of the tower, to provide a contrasting backdrop to the winding coil. Also, the pedal/power jack has an extra lead for the sewing machine light. I might hook up a small socket for a light mounted just under the platen so there is direct light on the coil.
One more thing I want to point out: instead of messing with reversing the motor, the whole tower is reversible! If you want to turn a coil the other direction, simply put the platen on the left, and the counter display box pivots to point the other way. The wire guide arm and the black square are attached with hanger bolts and wing nuts, so they can be removed reattached to face the other way, as well.
I'll probably get out in the shop later today, so I'll post complete pics.
-Ruining perfectly good wood, one day at a time.