3D printed angle gauge
A conventional angle gauge (such as the one above) can't measure the angle of an object with a sharp corner, because the hub of the gauge is the pin about which it rotates.
This design gets round that problem by having a rotating element that is constrained by its outer edge.
To make it, print the two files base.stl and rotor.stl. Also print the file protractor.pdf on a - well - on a printer.
Cut round the outside of the protractor print carefully with scissors and glue it in the recess of the base part. Take care to align the 0-180 line at the centre with the cross bar of the print. The best glue to use is mounting tack that comes in a spray can.
When the glue is set, use a sharp blade to cut the inner part of the paper away.
The FreeCAD design is in angle-gauge.fcstd if you want to change it (for example by making the diameter bigger or smaller). See https://www.freecadweb.org/
Thanks to the most useful website with all sorts of printable scales at: https://www.blocklayer.com/protractor-print.aspx . If you make this, slip them a quid or two.