This is my caliper. There are many like it, but this one is mine.

Rack and pinion from: http://www.thingiverse.com/thing:944471
Screw and bolt from: http://www.thingiverse.com/thing:1183

Note, the screw is left-handed, so that CCW rotation to open the calipers will tighten the bolt. This is intentional. When you go to screw it on, remember "lefty-tighty, righty-loosey".

Built in Tinkercad. Edit your own: https://tinkercad.com/things/1hsFTJBqkhA