How about using a garden-hose thread rather
than a soda bottle thread? Those gaskets are _really_
cheap and effective for both pressure and,
I'd imagine, suction.

In fact, I have used garden-hose gaskets for some backyard experiments
with soda bottles. They are pretty much an exact fit.

(One such experiment is to fill the bottom half inch or so of the soda
bottle with liquid nitrogen, screw a pressure gauge and seal on the
top, and then run away and watch the pressure gauge with binoculars to
determine the pressure at which the bottle explodes (circa 150 PSI). I
think I documented these experiments back in the 90's, probably on this
same newsgroup!)

I think the thread diameters are very similar, but a soda bottle is
8TPI vs a garden hose's 11.5TPI.

Tim.