This modified universal bottle cap can turn any bottle into a sinus rinse container. It uses the standard 'PCO 1881' threading as used in normal soda bottles like Coca Cola.

The only thing you need on the bottle is a ventilation hole at the bottom of the bottle to control the speed and stop flow by plugging it with your finger.

The cap can be printed without supports, since the bottom is bridged with one layer, which needs to be removed after printing (drill or knife - easily accessible from the bottom).
Print with the bottom on the bed - brim is advisable, since the contact area is quite small.

Use freshly prepared 0.9% sodium chloride solution (sea salt) and get well soon!

edit: The FDA recommends to use distilled water or boiled water (cooled down) for nasal irrigations to kill amoeboas, that might be present in tap water. In Germany the Robert-Koch-institute recommends this procedure for immuno-deficient patients, so it will obviously depend on the water quality to be expected, but it does not hurt to be on the safe side. Also draft medical guidelines actually recommend isotonic (0.9%) or hypertonic (2-3 %) saline solution.

• But don't take too much medical advice from thingiverse :)

edit2: Added a 1/2" threaded version upon request.

edit3: added adapter for 12mm inner diameter hose.