The plastic part of a curtain rails runner stop. Matches model 'SF' commonly used here in the Netherlands at least. See image

Two files included.
One for printing with support, which is the preferred one. If you hate support or want extra thickness, try the one without support. Make sure the piece lays horizontal in both cases.

Print in ABS, as it needs a bit flex to mount it.
use 50-70% infill

Prusa

i3 MK3

No

Yes

0.10

70%