I agree that the store bought devices to square a cabinet are ridicously expensive. I made my own years ago with 3/4 plywood in a few minutes. They are very satisfactory. Two months ago, however, I discovered very inexpensive aluminum semisolid squares of the “Swanson” type for less than two dollars at several post liquidation stores (Big Lots). They are  accurate at 90 degrees and can be easily modified to accomodate two clamps.