Measuring corner to diagonal corner is a common check for squareness in projects such as boxes and frames. Using an antenna from an old radio, I created a device to quickly check the diagonal dimensions of a project to ensure all 90° corners.
My antenna extended from 7.5" to 30". One end had a flat screw mount while the other had a protective button. I removed the plastic button and found a brass end which I filed to a point for more precise checking. On the screw mount end I fashioned a small corner attachment that allows it to be placed inside a corner or outside of a corner. A single screw attaches it to the antenna.
To check dimensions, extend the antenna to the opposite corner to determine your diagonal length. The antenna will pivot in the fashioned attachment while the extended total length remains the same. This allows you to move to another corner, pivot the fixed length and check for uniformity.
I found antennae in several sizes for $4 and $5 at a local electronics store.
