Karmakatie's right, just improvise with a slice o' condom for now. I haven't found one that fits either.
Yes, without the o-ring juice will seep up the carto, but it stays in the chimney. The o-ring is a little flimsy and gets scraped every time you screw in a carto. For some atties that get air from the side holes like the IKV i06 and especially the fatter IKV Platinum Atty, the o-ring must be out or there's no air. On others juice and air come in the bottom and the o-ring seals it and makes it work better.
I wouldn't play with that "screw" at the bottom. I don't think it will rise, but you could possibly break the connection, and it's an important fork so the contact can be made, while allowing air and juice passage into the carto's bottom hole. Just try to pull the i06 pin out a hair more, it should get there. Does it make connection on other cartos/atties?