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, I made a pair of simple tape holders. Each holder is just a scrap of ¾" thick plywood cut to 4½" square. Two pieces of ¼" Masonite 2" x 4½" are screwed to two edges of the plywood that make up a corner. Then, to hold the tape and allow for an accurate reading, the corner is metered to form a slot that fits the blade.
Ted's Woodworking - Guide eBook