Yup, there’s yer problem, mister! Looks like yer bearings are square, and ya need an 11mm nut lock, not this 13mm. I dunno what mechanic put this 13mm on here, but that was never the right part. We’ll get you fixed up right quick!
Thanks for the tip-off — I had manually grabbed a bunch of STLs, and until you pointed this out, I didn’t go back to the original list and see that there’s another part there. Gantry’s mounted! Woo!