Summary

Lid for standard PET bottle (commonly used for soft drinks) with hose barbs. It's watertight without any additional O-rings.

Instructions

Download the thread library http://www.thingiverse.com/thing:8793 and put it in the same folder with downloaded cap scad file before rendering the part. To get right preview in OpenSCAD, increase "Turn off rendering at ... elements" setting to 5000 (It's located in Preferences->Advanced).

Print two parts of the cap and fix them together with a superglue. Use a little more glue, than usual to fill any gaps, and be sure you have a ring of glue around a holes, or a part will leak.

Normally, the cap will be watertight. It's achieved by using conical seals. However, it's possible to put 21mm O-ring in the cap to get additional isolation.

If you intend to use this cap for taking water from inverted bottle, use a second hose barb to connect a piece of plastic tubing inside of bottle to let an air in, or create siphon the same way.