While putting my XXL Pro together I kept checking square as I tightened. I have heard of people using a ratchet strap or rope to keep pressure the direction they needed for square.
Hi LH, looking at your picture I notice that the one screw is butt against the back of the countersink and the other against the front. This could be an issue that can be fixed by loosening both extrusions and start tightening screws from the front towards the back. Hope that works.
Turns out it was just a full quarter inch out of square haha. The printed instructions didn’t specify to measure diagonally and nudge it into squareness. I thought moving the gantry from baseframe to baseframe would square it. Went back and followed the video instructions and that did the job.
I used an engineer’s square before I tightened everything and it was square on my first measurement after that. Every board extrusion lined up straight with the same gap.