Back in the good old days when I didn't have access to a table saw, I used a straight piece of 1/2 inch plywood, measuring tape, and c-clamps to make my cuts. The best boxes were built right before I entered college because I worked for a company that built portable offshore quarters, and they let me use the huge radial arm saw after work or during breaks. If only more people would have been using fiberglass in 1991, I could have made some wicked enclosures!
Back to the topic at hand, my first box ever looked about like yours. I removed all my drywall screws from it and set it on fire when I rebuilt it.
Now, I am so lazy, I just pay someone else to build me an enclosure...