Armorer's Calipers
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Maximum measurement: 23 1/4".
Arm dimensions: approx. 330.2mm x 177.5mm x 5mm

The joint is made of a peg and through-hole, meant to be riveted. All the sources are included if you want to make a threaded joint instead.

I've included the SVG file (built in Inkscape), DXF v14, OpenSCAD, and STL files. This was a quick hack job - you can see that I even got the spring notches backwards in my test print. You can probably get another inch of measurement by smoothing out the inside curves. I'll add a lead screw and nut to a later revision.