Each holder consists of a square block of plywood with two pieces of hardboard attached to adjacent sides, as shown in the drawings. The hardboard laps over the edges of the block on both sides so that the holder can hook over the corner of the assembly. The adjoining ends of the hardboard are mitered to form a slot that can hold the end of the tape or act as a "cursor" for your measurement (drawing and photo).
The holders make taking the two diagonal measurements quick and easy. One minor regret is that I no longer have any excuse for an out of square assembly.
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