Slides over the aluminum rail protrusion of a Kryptonite lock and latches on it.
I keep my lock in my bag so the rail isn't useful but damages the frame's paint when locked.
I printed mine in PETG and they hold well. I don't know if PLA would work as well because of the wide range of temperatures outdoors and because the part needs a little flex for the tab to latch. Although if you don't constantly remove and put the sleeve back, it shouldn't matter too much.
Tested to fit on the grey u-lock and the orange u-lock models.
The author marked this model as their own original creation.