Since the RinseKit Pod is an older discontinued model, I thought it would be worthwhile sharing an end cap in case someone were to lose or break their original. This does require you to use the O-ring that comes with it. From a quick measurement, it looks like it's a 25 inner, 33 outer o-ring. From experience, TPU printed O rings do work is some applications. 

The reason I designed this cap was so I could edit/drill a hole and attach an air valve to add pressure while camping. The plan is to do this with a bike pump.