This is one of my favorite tools:

https://www.amazon.com/Johnson-Level-Tool-Stainless-graduations/dp/B00004T7ZV/ref=pd_lpo_vtph_229_lp_t_2?_encoding=UTF8&psc=1&refRID=1CTFQQRATGV1TEWE9E6G

It doesn't open and close quite like calipers, but you can buy 2 and rig one up to have 2 of the slider clips on it and pretend :).