Comparing the diagonals of a cabinet or a box is a great way to check it for square. But when you're working alone in the shop, measuring long diagonals accurately can be difficult. To make this job easier, make a pair of simple tape holders.
Each holder is just a scrap of ¾″ thick plywood cut to 4½″ square. Screw two pieces of ¼″ Masonite 2″ × 4½″ to two edges of the plywood to make up a corner. Then, miter the corner to form a slot to hold the tape and allow for an accurate reading.