This is going to be quite an adventure!
Try to design such that an o-ring or gasket will provide your sealing surface, 3d printed threads [especially printed vertically] are likely going to let you down. There may not be enough teflon tape in the world to make that work.
If you print with 6 walls, 6 tops, 6 bottoms, it will give the interior part a good chance of being waterproof. pressureproof? hmm