This is a wheeled trolley that is designed to fit into strut channel (Unistrut). The trolley is designed to accommodate standard 608 bearings. (I printed my bearings from here: https://www.thingiverse.com/thing:4821342 ... awesome work on those bearings). The trolley also has a hole in the center to accommodate an 8mm bolt.

In my application, the strut channel is mounted on the ceiling and the trolley rides on the rails. The center hole allows me to bolt a hook from the bottom of the trolley.