If you don't have them already, I would suggest a good sized square, and a 6 foot straight edge. Those tools don't cost too much and allow you to find where the error was made, if any. Boon could be right on in that the floor it's sitting on isn't level. Other than that it looks like a great stand, you did everything right, good job!!!
Worst case you have to take a few pieces out and re assemble if it is truely out of square, stick with it, your really close to having a killer stand.
GOOD JOB!!!