Temporary schtemporary, it might work better than a new o-ring. The sizing is always such a problem, a fraction of a mm too fat or too skinny and it's a miss. So check the rubber for nicks first and cut those off with a razor so they don't catch. Then try dental floss under your existing ring, or maybe a fine strip of plumber's teflon tape, those might be a real solution.
The seeping that occurs after filling, when you open/close the juice control platform, is to some degree inevitable, IMHO. You're pushing/pulling on the whole juice+vacuum+surfaceTension apparatus. If I look inside I find there's always a coating of juice down in A3 - so what. You can turn the KF on its side, let the little puddle gather and suck it out an airhole, boy that sounds degenerate. If it's just a smear, I just wipe it, toss the Kleenex, and go back to the rigorous business of living da life o' vapin'.