Wow, that's awesome! I've been looking for a small canister filter for my 10G tank but couldn't find one and stumbled on this looking to see if I could make my own. There really is a lack of small canister filters on the market. My Hagen HOB filter also seems to get rid of all the CO2 I put in the tank and produces way too much current for shrimp. What size powerhead do you use?
I was thinking of using something similar to the homemade CO2 reactors made out of a Python nozzle but filled with filter media, but this looks like it could be easier with the right tools.
Could you post a parts list and maybe just a short list of steps to build this?