Answering question for Corners - Fender, for amplifier, Steel, 3-Hole with hardware

Product:
Corners - Fender, for amplifier, Steel, 3-Hole with hardware

Question:
How do I measure for the corner brackets to see if they will fit without ordering a set? I purchased a hand-made guitar cabinet. This is the part I am considering that is in question. https://www.amplifiedparts.com/products/corners-fender-amplifier-steel-3-hole-hardware