Flow - Using the walls of the basement for walls of the box is not a good idea. Aside from the fact that it would be harder to build, you have a cold, dank wall inside your box and that's not good.

I'm going to be building a box very similar to what you want, starting in a day or two. I can do a DIY on that box and you can follow along if you want. If you want to get started right away, What I would suggest is that you have a floor board cut to exactly the size you want, then build a 2x2 frame on top of that. Then have your side panels cut, attach them, measure again for the front and back and have those boards cut. The reason I suggest having the plywood cut in stages is that you need to overlap the corners and you won't know the exact measurements until you have the side walls up. As a final step, have the top panel cut. (Depending upon a number of factors, the top piece may or may not be the same size as the bottom piece.)

...or you can do like Roughrider said and buy a pre-fab cab.

PC :smokin: