Here's an easy way to square up and glue any type of cabinet or case. This method uses an insert made from a scrap of plywood that’s sized to fit the inside of the case. To provide room for glue squeeze-out, I trimmed each corner of the insert at an angle.

To help hold the insert in place while you add the clamps, screw a pair of cleats across the top of the insert. After setting the insert in place, tightening the clamps squares the box around the insert.