I must have a dozen pinch rods of varied lengths, and materials. Most have a variety of ends to fit into corners, around sides, and so forth. Possibly one of the most accurate ways to measure for square from the inside, or distance from anywhere, especially inside measures. Easy to make your own, and there are a number of "clamp" features to buy, and or make.

Lee Valley sells a nice one if you prefer to just buy items. A hint, you can buy square brass, or metal stock from 3/4 x 3/4 and larger from the Borgs, and places like McMaster Carr. Drill and tap for a set screw, and cut hardwood arms to your desired lengths. Cut an angle, or make a wedged end, and go forth and check measurements.