why dont you finish the build before you worry about such things. none the parts we use are perfect, thats called tolerance. there is plenty of room for error ther way ryan designed it.
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.