So I found a solution.
I squared up the frame, and mounted everything without the center assembly.
I tied the slider that didn’t quite meet the corner into place with some string.
Covered the sliders with tinfoil.
Heated the metal tube with a hot airgun near the sliders.
After 5 min or so the slider not tied in place can be twisted until it is no longer forced into place.
Let it cool, and re-tighten the screws.
All nice and square,….