Here are plans for a simple caliper, handy to have around the shop for measuring thickness and diameters. Anytime I can build something that helps me build something else I am always pleased.
The plans should be as accurate as your cutter or printer is, I actually used this project as a calibration for my laser.
If you don't have a laser you can print the PDF and glue the paper down to any flat material and cut it by hand.
In the case of a laser, I engraved the markings and painted them, then sanded off the extra paint, for more details on the method see: Laser Engraving with Highlight Paint.
After the paint is dried and sanded, assembly is simple. The lower caliper arm is bolted to the other sliding parts with 4- m3 bolts. The lower caliper arm has a few millimeters of adjustment space so you can change how tight the slide is.
Check out the plans and details at 4volt.com/projects/calipers/