tl;dr: dr won't hurt

These three iterations of the evolving "top" part show the same sag around the X motor:

So that's been happening for a while. I've let it go on without much thought because it hasn't caused much trouble.

Part in context:

That was a predictable weak spot, analyzed by the method of build-it-and-see-how-it-breaks. The part gets thin over the motor frame and adjacent bearing clearance which weakens cantilever support for the motor. The left and right sides provide better cantilever support for the front corners which support the front edge which would better support the front side of the motor frame if no big chunk of ventilation were cut out from it. Tying across the top and bottom of the vent opening resists bending along the front edge, but the resulting parallelogram does little to resist shear. Diagonal bracing across the vent opening might help, but obstructing airflow doesn't feel like the right solution to a heat problem.

So stuff sags after a while. This has two effects:

1. The motor and motor end of the screw drop. I've assumed imprecise vertical separation between the layers of the XY stack from the very first try at the basic idea, so tolerance for a little droop of the motor+screw unit was already in there, and proves more tolerant than expected. This has been a non-problem.
2. Deformation relaxes the zip tie around the motor end of the motor+screw unit. This has been mostly a non-problem because that mostly doesn't affect axial constraint. Once (maybe twice?) the tie got slack enough to let the corner radius of the motor frame drop to the level of the relatively shallow end of the pocket into which the motor fits (maybe that needs a picture...) which let backlash grow enough to matter when I wanted really small backlash. So when mostly-not-a-problem got to be enough of a problem to notice, the fix was to replace the tie holding up the motor.

Flipping the motor down to the top side of the more substantial "middle" part would be easy, but that would expose the screw to falling debris, which I've been able to avoid so far.

A while (um, half a year by now...) ago I noticed that the sagging front edge of the "top" part was in solid contact with the "middle" part. I guess that explains why all three examples above had sagged to the same extent.

That apparently is also a non-problem because, even though the parts are firmly in contact through about half of the slide travel, the rounded part of the middle part has no edge and the bottom edge of the top part is burnished smooth. Where "firmly in contact" means firmly pinch and hold a sheet of paper forced in between the two parts. The fact that it all works fine shows that this makes a sliding interface with very little friction. Maybe I could sell that as clever mechanical design if it wasn't so obviously not.

So... nothing. If you're thinking about building one of these for yourself, don't stop for this.

What's not in this log update includes sequelae of discovering unintended sliding contact.