I love using a thin tape measure. It works, has a lock, and is skinny enough to get between terrain and minis.

https://www.amazon.com/gp/product/B008AGWOXW/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1

I personally bought two, so each side could have their own.

All this said, my campaign mates are pretty chill with measuring. We're all fine with holding it above tense action and eyeballing.