Race Face AR 30 asymmetric rim Presta valve spacer

The shape of the rim makes it difficult to seat and seal the tubeless valve properly. 
Can also work with non tubeless valves, just for better looks and support, or color coding/customization.

I have printed it in 100% infil ABS 0.08mm and it fits well, but probably PETG and PLA, or anything else will work - just print it in the highest resolution possible.
For 100% perfect fit put some low grit sanding paper on the contact surface of the rim and give the spacer few swipes to remove some of 3d printed steps/high spots.
Other way is to just print in TPU for more compliance.