Ok, I just made a one way valved lid, but used a ring of soft rubber tubing as the grommet (because I didn't have any real grommets). By jamming the fairly hard plastic of the check valve into a hole drilled into the lid and compressing the tubing a bit, I think I have a good seal. Certainly the amount of O2 getting back in the jar should be minimal given the positive pressure of the escaping CO2. Also, the hole in the lid is sharp enough to really dig into the hard plastic. It definitely doesn't want to come out without a fight. In the past I've just kept the lids loose as SYP suggests, but for camping/traveling, I like the idea of being able to make the lid secure. Thanks kevsnova.