Honestly yep, drywall square works fine. In my pre- disposable income days, I'd use a long melamine shelf board clamped to whatever I was cutting to use as a fence to establish one straight long edge. Then I would use a shorter shelf board with a framer's square against that straight reference edge. It's the $20 solution that does the exact same thing as the $2000 solution in terms of straight edges at 90 degrees to one another. It just doesn't clean up after itself.