Best advice ever:
Regardless of what you're doing with your cab, be it fancy as all hell, or just a box with a TV... Take your time.
That's it. Measure out a peice, draw the lines. re-measure again, and again. Use a speed-square and make sure your pieces are square. Then re-measure again. THEN cut...
I heard the above advice upteen times. I measured twice, no three... actually four times, and I still ended up with some bad pieces.
I've been at my cab for the better part of a year, off and on (had a pesky wedding to take care of in the middle there)... I'm not even halfway done yet. No matter how carefl you are, mistakes will be made until you get the hang of things.
For some more specific advice: Whatever you do, make sure that the "side" pieces are the exact same size. Basically, cut them both out, clamp one on top of the other. Using a router and a flush-trim bit, route one to the exact size as the other. It'll make life a lot easier when you start to add the "center" pieces and you can be assured that they'll match up perfectly to the sides.(no "lean" to one side) if your pieces are all square.
- FA