Well, I think that photo provides a lot of information. Good size reference, and very likely the reason why you mounted the stepper motor the wrong way round. LOL

You can buy packs of those aluminum channel nuts on ebay or aliexpress, just double check the size & style you need. They usually come in M4, M5, & M6 sizes (M4 & M5 more common) and the style I believe you have you can drop in from the front of the channel, it will then self-lock after half a turn. The other style has to be inserted at the end of the channel and slid into place, which sucks if there is already something else mounted.

That calibration kit you describe, does it possibly have more components to it? Under the bed perhaps? Like FSRs? Or is it just the contact switch setup?