Gasket for Rich Earth Institute Urine Collector

If you don't want to flush your pee with some perfectly good drinking water, and maybe even turn it into fertilizer for plants, you can make a DIY pee collector following instructions from the Rich Earth Institute:

All the parts for this project are available for purchase online or in stores, except a custom made gasket that creates a check-valve in the base of the funnel along with a ping-pong ball so that urine odors don't float up into your bathroom. Rich Earth Institute cuts these custom on a CNC machine and suggests you get a similar sized washer.

Well, it's Saturday so the hardware store is closed, and I don't have a CNC machine.

I reverse-engineered their CNC cut part and modeled it in OpenSCAD.

I recommend printing this on a smooth bed and placing the part upside down in the funnel (flat side that was touching the bed facing up). This will ensure a seal with the ping-pong ball. Hmm printing with concentric top/bottom infill might also help with this, but I didn't do that.