Doe's anyone know where I can find directions to build my own caliper?
Are these what you are talking about?
If so, no directions needed especially. Get some long enough drawing paper.
Draw out the shape of your calipers, cut them out with scissors and see if the shape accomodates
what you need. Once you've perfected the paper model, your ready for wood .
Press wood, fiber board is highly preferable to plywood. 1/4" thick is adequate., dont go thinner.
Calipers need to swing closed but still have a pretty good opening near the rail area.
Need a jigsaw and maybe drill and access to a hardware store.