$13 8020 T Slot Aluminum Joining Plate 40 S 40-4325 N
Niki at lumberjocks.com
shows how to make a Bar Gauge (or simply - Checking diagonals) in this Forum Topic
The “bars” are made of 1/4” thick pine. The “fences” are made of 1/8” pine but plywood is ok The round edges are cut from curtain rail but a dowel or broomstick will do.
I made the fences so I can hold the bars with one hand and clamp with the other hand.
I made the round edges after I got tired to clean the glue squeeze-out from the “Schpitz” (German for apex) of the bars.