Cap for MSR dromedary bags with an opening for a valve. This valve was found with some abandoned trash in the backcountry, but I think they can be found in some boxed wine packages (it can be removed easily by placing in boiling water).
This cap is compatible with Nalgene, Camelbak, and other water bottles with the same thread (though I'm not sure why someone would use it with one of them).
I used WoodWax on the plastic for improved water repellency and food safety. Chapstick might work as an alternative, but I haven't tried it.
Print oriented as shown in the preview, without support.