How about this:
Get some 3/4" plywood. B/C should be good enough. You should only need a 4x4' sheet, but you might only be able to get an 8x4' sheet.
A. Cut two 16"x16" square sheets.
B. Cut two 12"x16" sheets.
C. Cut two 12"x17.5" sheets.
The A sheets are your front and back.
The B sheets are your sides.
The C sheets are your top and bottom.
Route a 12" circular hole in one of the A sheets for your speaker.
Drill a hole in the other A sheet for your 1/4" jack.
Attach the B sheets and the C sheets to the speaker sheet. The easiest way to do this is to use edge-to-face butt joints and screws. For stability, use a square or triangular glue block at the joints for stability. You now have a box with a hole in it.
Attach the speaker to the hole. Attach the 1/4" jack to the drilled hole. Attach the wires from the speaker to the jack. Attach the jack sheet to the box.
There ya go.
If you are handy you can use rabbet joints or, even better, box joints or dovetail joints. You can also experiment with the size. For example, make it long enough to stack on top of your 4x12. As an alternative, you can make the jack sheet half-size for an open back design.
A big point of this post is that there isn't a lot of rocket science in speaker cabinet design. The classic Marshall 4x12's were based on practicality, NOT acoustics.