measure for box

OK sprawl, here's how you do it.  Go to your local lumber yard and purchase four full sheets of 3/4" MDF.  Lay one sheet on your workshop floor and very carefully mark out your 12 X 12 square exactly in the middle of the board.  If you are not sure how to do this, purchase a 12" square cardboard box from a local U-Haul store and use it for a template.  Cut it out, being careful to cut on the outsides of your lines so that you end up with the correct size.  Now glue one end of your 12" square to the exact middle of the second sheet.   Clamp and let it cure for 24 hours.  After 24 hours, remove your clamps and mark a line the entire length of the sheet exactly perpendicular to the edges of the 12" square.  Rip the sheet along these lines on a table saw.  This way you'll be certain that the edges align perfectly to your bottom.  Then turn the piece and cut off the excess MDF that extends from the bottom side of the 12" square base.  Cut close to the bottom, but leave a little extra.  You can trim this later with a flush-cut router bit or, if you don't have a router, a nice flat hand-rasp.  Next, measure up from the inside of your base to the proper height for your desired sides and cut off this MDF.  You will want to save these pieces as they will come in handy later.  Now glue and clamp as before the opposite edge of the 12" bottom to the third sheet of MDF and repeat the above steps to form the opposite side.  For the top, you can use part of the left over from the second or third sheet.  Just lay it across the two short sides, glue and clamp, wait 24 hours, then cut and trim.  The same process can be used for the front and back.  Assuming you have followed this process, you should now have a nice-looking 6-sided box.  If you screw up, that's where the fourth sheet comes in handy.  Take it to a local car audio shop and see if you can barter for some help to build your enclosure.  Work slowly, be careful, and before you know it you'll have spent a week and $200 on your box.

Oh, and this is for you: