I'd definitely advise cotton. If you really want to use silica, then 3mm is probably too fat. Try 2mm, double it back from the wick slots, and tuck the ends down into the deck. If you're going to use the o-ring, slip it over the chimney as described above - but with cotton, I've built it with and without the o-ring, and it doesn't seem to make a lot of difference. FWIW, I had the exact same problems with the Fogger at first, but after a couple of days and a lot of experimentation, it worked, and worked very well. Perseverance!