Hey Jim,
I had a look at you video. Really annoying, but the problem is created by the machine on which these parts were machined. So they may be all out of square in one batch of QCW frame, perhaps in all ever produced (*).
I think just inserting a shim would be a too small contact surface. What you would need is a total metalworking artist with a hand file who files everything to an accuracy of .05 mm, or re-milling everything on a CNC milling machine where the milling head cutting edge and profile feed are REALLY at right angles.
The other way would be to relocate the holes for the feet bolts on one side of the machine. You would still not have the t-tracks rectangular to the other axis, but who cares for t-tracks, they’re just there for clamping. The only thing that counts is if you mill a shallow line with the tip of a v-bit around the entire workarea, is that the X and Y are rectangular to each other. The milling path, not the the t-tracks. They don’t matter as long you don’t take them as reference. A reference would always be a fence that you mill on the wasteboard AFTER X and Y axes are rectangular one to the other.
*) Footnote: DISCLAIMER: These are just assumptions. Yet I did not assemble my QCW frame. It’s still on the list for when I will have moved to a new workshop.