The Dupla type S reactor I have is a little different concept. With it, the water goes through the CO2, not the CO2 through the water :-)
Imagine it as a plexiglas cylinder, about 25cm in height and 6cm in diameter. It is filled with the small type of Dupla Bioballs. Actually, a miniature trickle filter. Both CO2 and water (from a pump) go in through the top. Initially, the cylinder is filled with water completly, but as more CO2 gets injected, the water level in the cylinder drops and the water trickles through a pure CO2 atmosphere. The water goes out at the bottom of the cylinder. Should be faily easy to DIY. Disadvantage: some gases other than CO2 which might be present in the CO2 source can't escape and will eventually accumulate if they are not readily water soluble. So the cylinder will have to be flushed from time to time.