food-safe silicone sealant works...
mason jars (pressure and rubber o rings)
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Anything in a freezer bag can be made airtight by lowering the filled bag into water. This pushes all the air out. Lower it to a fraction from the top, and then seal it. A good freezer bag should have an airtight seal.
(Though Lemonie's approach is probably the right one: The best answer depends on what it is you're actually trying to accomplish.)