You’re supposed to secure the photo backdrop’s top crossbar to the uprights by fiddling with a wingnut, which you must do while reaching over your head. Emart apparently realized this operation was fraught with peril, because the package contains four wingnuts. After setting it up once, I replaced the wingnuts with finger-friendly knobs containing acorn nuts:

The upright pole ends in an M10×1.5 stud, which fits the biggest acorn nuts in the Warehouse Wing.
The knobs come from Thingiverse, although the OpenSCAD program required a bit of rework to make it compatible with the current version. Fiddling around with the Customizer parameters produced a Good Enough knob:

I pulled the acorn nut into the knob using the upright pole hardware to keep it aligned. Spin the wingnut on the stud “backwards”, add the washer, push the nut slightly into the knob to get it started, then thread it onto the stud:

Turn the knob to pull the nut inward until the stud hits the inside of the nut:

Unthread the nut a bit, run the wingnut out to meet the bottom of the knob, and repeat the operation until the nut bottoms out inside the knob:

Toss the wingnuts into the Warehouse Wing against later use.
Bonus project: on the other end of the upright, you’ll find it impossible to actually lock the leg carrier against the pole:

The plastic fitting is … generously … sized around the 25 mm OD upright pole and requires more compression than I could produce with my puny fingers. It turns out the 18 mm OD leg tube exactly fills the space available inside the fitting, so you (well, I) must squash the steel tube in order to close the fitting on the pole.
Remove the wingnut + screw to free the end of the leg, stick an inch of the leg into the bench vise’s soft jaws, and mash gently to about 16 mm across the holes; it’ll expand slightly in the other direction. Reassemble in reverse order and discover the thumbscrew now squeezes the fitting exactly as it should.
There might be more finishing to do when we actually hang a quilt from the stand, but at least it’s now usable.
Sheesh and similar remarks.