You may need to position a clamp diagonally to square the box.
Woodworking joints for boxes. Another common wood joint used in cabinetry is the rabbet. A rabbet is essentially a dado cut along the edge of a board. Dovetails are considered the hallmark of fine woodworking especially hand cut dovetails. They are used in fine woodworking and also in production especially drawers.
A basic butt joint can be strengthened with hidden fluted dowels or a rabbet butt joint can be cut first dowel holes can be drilled and contrasting solid dowels can be inserted and cut flush. Clamp the box together as shown below. Box generally machine cut box joints are very strong due to the large amount of glue surface. This type of joinery works fine for making box like structures such as drawers.
Rabbets are often used at the back of cabinets and other similar assemblies for attaching the back to the sides of the box adding a considerable amount of strength to the assembly. They are also very strong visually and if used. While this method has a reputation for being slightly less durable than a dovetail joint it s been used successfully for a long time and is probably the easiest reliable method. To join your box pieces apply glue to all of the mating surfaces with a small brush.
However you ll need to be a bit more diligent about keeping the box square when clamping than you might need to with dovetails. To assemble simply apply a thin layer of glue on all joint surfaces and slip the joint together and clamp as needed.