I’m using regular 3/4" EMT. Larger EMT or 45 degree square angle would all have the same tolerance, which is that the highest point must be within the width of the 608 roller bearing. The brackets for holding the tube (made for the right side) have room for some adjustment in the X direction, so the size of the frame doesn’t have to be perfect within 1 mm. (And a modified bracket can be made too if it’s still too far off.)
I am also thinking maybe next time the tube mounts could be mounted to the face of the ribs in this type of design:
The attachments for the ends of the belts will still need support, so the side slats can’t go away completely, unless the belt ends were mounted to the ends of the metal tubes somehow…