I needed a quick measurement of a square(-ish) hole in a chimney for a cover and drew this up quickly.
This tool measures square holes between 200x200 to 300x300 millimeters using its outside perimeter.
Its wide edges allowed me to apply some extra tape to indicate curved/non-straight hole borders.
I've added some corner chamfers to account for any fillets or rounds that are usually found in square holes (in buildings).
Although the printed dimensions are absolute, for more precise measurements it might be better to use this tool to gauge the hole and then get the dimensions off of it using e.g. a tape measure or (big) calipers.

The 4 springs can be glued in place but should also stick without it just fine.
Before assembly the narrow legs of the sliders are quite fragile, so handle with care.
Also, tolerances are a bit tight, so it might take a couple of strokes (or drops of grease) to get it moving smoothly.

'Ironing' the top surfaces might improve sliding and/or legibility of the texts.

Finally, while assembling I noticed that it might have been better to flip the springs around to make them into kind of an end-stop preventing the sliders from being pulled out fully by accident. Maybe next time.