Right trapezoidal nut too tight
The right trapezoidal nut was so tight that it completely prevents the right Z-axis motor from working. At first I thought perhaps it was a bad motor or cable, but when I remove the trapezoidal nut the printer works fine (other than the right side hanging down a bit). So I'm sharing that in case it's useful to anyone else.
So I'd like to replace it. It's not a printed part, but I did find https://www.thingiverse.com/thing:2246502 which is for the Mk2. I figure I'd give it a shot (Nylon is cheap) but I'm not sure if the Mk2 and Mk3 are the same for this part. I'll know more tomorrow. Though I imagine a non-printed nut would be more reliable in the long run, I just got my Mk3 assembled, and I'd love to get it productive sooner rather than later.