In response to all the sculptors who've asked about this tool, a short description for building them.
The Calipers are made out of aluminum stock found at many hardware and hobby stores. You'll need about 2 feet. It's 1/2 inch by 1/8 inch stock Each piece is 2 inches long including the needle holding ends. You can see how I ground those down to a 1/8 inch end, and into that end you carefully drill a 1/16 inch [or slightly larger] hole at least 3/4 of an inch into the end. This is so you can get a BIG needle [like a carpet needle] and epoxy it into that hole for the pointers.
The thing that makes this work is that you use LOCK nuts, so they'll hold tension. The black washers are common FLAT sink washers for a sink. Once you set the tension of each bolt to what feels good to you, this combination will hold that tension. Every 2-3 years you might have to tweak that tension a bit, but mine have worked for years.
I suggest the use of a drill press and grinder for this, but a regular drill, file and vise will do. [remember to tap all holes for predrilling].
Hope this helps some people. I designed them out of frustration with regular calipers, and they will reach around just about any point for measurement.