I wasn't satisfied with the stock acrylic spool holder that came with the 2020 kossel mini kit from folgertech. It was sticking out quite a bit more than needed, and made of two parts. Additionally I wanted to feed the extruder motor (which I mounted on the lower rail with a bracket I designed for that purpose) in a more direct path, so the arm is rotated 30 degrees.
I printed this upside so that the support structure is on the top. It leaves a little bit of a rough surface but that doesn't affect unwinding of the spool at all.
When mounting this, it snaps on the rail. For added security, I used an allen hex key in the channel behind the holder to keep a T-nut at the right height, then screwed in an M3 bolt.