This was a fun one! A flap valve in two parts, already assembled and ready to go right off the print bed! My hardware store didn't stock flap valves in sizes to suit small tubing, so here we are. This is sized for "15mm" PVC tubing, which has an outer diameter of about 21.3mm from my supplier.
Now, the the fit of the hinge is intentionally pretty loose, since it needs to flap back down after being pushed open by water. This means it should scale to some degree if you do want to resize it. But hey, it's 3D printing, and this is a pretty quick print, so experiment and see how it goes :)