I finally got sick of the bad o-ring fit, and found some that fit really well (better than what ships in the parva tanks).
I tried a slew of different sizes at work (we have huge bins of various o-rings for past product prototypes) to ensure that these were the best fit possible. The other advantage is that both top and bottom rings are the same size now, making for easier replacements.
If anyone is interested, they are a non ASME standard size on applerubber.com, size 0.347 x 0.059, 70 durometer Buna-N.