Inside and outside calipers
thingiverse
Inside caliper length ~14cm, outside caliper tip to tip ~15cm. Both at 3mm, but you can always increase the thickness. These calipers have a tapered tip to help in pinpointing your measurement, but the downside would be a weak end. Note that you require some screws(M3), washers and nuts to secure the prongs.
Download Model from thingiverse
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