Fits on the end of the focus knob, not a perfect tight fit so you can remove it easily, but it fits to the contours so there isn't any give when moving it and stays on even when you're pointing at the zenith

I found existing ones either too flimsy or made for the 102/127/180, not the red headed step child, the 150. If you want one that is a really tight fit, 0.5% off the size will do it. - It's got a 1mm difference between the bottom and top to stay in place but still go on easily, bear in mind if you do scale it.

Printed in Nylon, no idea if that affected the sizing, test print was in PLA and seemed fine too.