“Pinch rods” or “pinch sticks” are, at their most basic, two thin pieces of pointy material that are held together loosely enough at the center to move, but can be locked down. Put a point corner to corner in a carcase, lock the sticks, then check that setting against the two other corners. If they match, you’re square. If they don’t, you can tweak the carcase as needed.
You’ll find lots of options online for making your own pinch rods, but this stainless steel and zinc version collapses to fit inside even the smallest tool chest or drawer (the tool is 7″ long with the halves unscrewed).
It works not unlike the TV antennas many of us remember from our childhoods – but the metal is more robust.
It would be better, however, were it a bit heavier gauge, still. If, like me, you sometimes lay a carcase on its side to check for square, the tool can bow a bit when extended if you don’t support it at the center. That said, fully extended the EZ Pinch Stick is 92″ – that’s a lot larger than most furniture work, so you can overcome the bowing by extending the wider-diameter tubes and leaving the thinner ones collapsed.
It would also be great if the points were a bit pointier – an easy fix at the grinder – so as to fully fit inside acute corners. Still, it’s a clever, space-saving idea; my only reservations are easy to address.