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.
Draw circles for the speakers on the front panel of the cabinet with a compass or large protractor.
As guitar speakers are different so their cabinets are different.
Primarily the cabinet is a convenient way of housing the speaker so it should be solidly built to ensure no joint vibration unpleasant buzzing and be strong.
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.