[Rikko]

Find a small fiberglass coffee filter - the ones that are cup-shaped and made of a very find mesh. Tip it upside down and let CO2 collect under there. It won't bubble out unless the whole chamber is filled to brimming but CO2 will diffuse across it. I've yet to set mine up in my tank as I've been busy with reef stuff.
I did have good luck using a plastic tube (I used most of a gravel vac) with a big chunk of foam stuffed into the top. Again, the CO2 built up inside the foam and the CO2 diffused into the water without bubbling up, albeit there was the odd "fart" when too much buildup occurred.