Designed to fit Nalgene style widemouth bottles (including some CamelBak bottles and MSR Dromedary Bags)
Optional gasket may be necessary to get a tight seal, it depends on the design of the bottle, materials used, print resolution, etc. The gasket should be printed in a rubbery material like NinjaFlex.
I used Knotwork Woodwax to coat the inside of the cap (that's my sister's site). It's not designed for plastic, but I find it adds a water repellent and food safe coating.
Print upright the cap upright, without support material.
Update Oct 1, 2014: Added a new version this one should be easier to twist and is more likely to work without an additional gasket.