An old WD-40 can with the top cut out to drop the vent line into.
The can vents to atmosphere.
^{1) 2)}

Designed and sold by RW COMPANY'S BIKE STUFF ³⁾.
Here's a video from the maker on how to install it.
This one is installed with the addition of one 5/16“ hole in the sprocket cover, some 3/8” hose, and a brass tee. ⁴⁾
It's basically a catch can with a filter on the top.
^{5) 6) 7)}

^{8) 9) 10) 11)}

¹²⁾