Once the glue is set, use the plane and chisel to remove any protruding wood or glue.
Register the plane against the edges of two sides and take a shaving off the rim of the box. Do this for all four corners to make sure they are flush. It’s important to keep the plane as square to the sides/ends as possible to prevent any sloping to the outside.
Repeat these steps on the underside and mark it so you know which side to glue to the base.