Cut two pieces of plywood or mdf to 28 inches by 28 inches for the front and back of the 4x12 cabinet.
Build your own 4x12 guitar cabinet.
Back in the 90s celestion put together a comprehensive guide to designing and building your own cabinet the cabinet handbook.
The box design is acoustically less critical than that for hi fi or pa systems but proper construction is essential.
Draw circles for the speakers on the front panel of the cabinet with a compass or large protractor.
Cut two pieces to 14 by 30 inches for the top bottom and two pieces 14 by 28 for the two sides of the cabinet.