I based it off these instructions:
http://www.qsl.net/w2wdx/aquaria/diyco2.html
scroll aaalllllllllllll the way down for the powered reactor. The rest of the article is really good too tho.
Since my tank is only ten gallons I used a small internal filter I had laying around (the elite mini, 25 GPH, not sure on $$, came with a cheap 2.5 gal), an airstone I had (cost $0.89) and a small squirt bottle ($0.99).
Basically I cut a slit near the bottom of the bottle and forced the flattened "v" shaped nozzel on the elite filter into the bottle. I cut the neck off the top of the bottle. Then I carved a hole near the bottom, shoved the airstone's nipple in there, and attached my CO2 line. I then covered about 90% of the bottom with filter pad (just rubber banded it to the bottle).
I had to experiment with the placement of the pad. Mine's kinda dense so water wasn't coming out of the reactor too well. I adjusted it so the CO2 heavy water could exit and circulate through the tank, without letting many bubbles out.
It took less then a 1/2 hour.. looks kinda like hell but it works. This was kinda a proof of concept for me, since I had the parts already. If I feel the need I'll probably rebuild it in the future with a better PH. I'll try and get a halfway decent pic of it in the tank.
EDIT
Here's the pic. Basically CO2 comes in the bottom, rises against the current to the swirling bubbly vortex up top, and dissolves in the water. Water + CO2 comes out the bottom.
In the background you can see some of the bush algae that drove me to make this piece. Hopefully greater CO2 levels will help.