I there a printable jig for squaring the outer frame. I have not seen anything in thingiverse but I might have missed it. I remember that I used my square with a caliper last time but that was far from ideal.
NM, I found something. Its not idea since its for 25.4 but should work for my purpose.
I just used corner to corner measurements and verified they were within a couple of mm of each other. Other than that you might also measure height at each corner to make sure you have your legs fully seated.
I used this. it worked pretty well. I am sure that I could have designed something but that always takes a lot of time where this is already done: