Cut about 1/4" off the white plastic nipple that the brush head attaches to.
I used a metal clevis cut in half, 5/32" brass tubing, and a threaded coupler. Solder the threaded coupler inside one end of the brass tube and the toothbrush to the other end. The saw blade gets soldered to the half clevis and screwed to the coupler. You can make several attachments this way and switch them out.