Simple radius gauge, from 10mm to 3mm in 0.5mm steps, you can measure convex and concave forms with the two opposing ends.
There is a hole for holding them together too.
I am thinking about a measuring hole for rods/screws for the smaller sizes too.
Be aware that your plastic will shrink and the forms may not be that exact then, but if you use it for measuring forms for your 3d printed parts that may even be an advantage ;)
Print either the complete build plate (build_plate.stl) or the individual parts you want to use.