The easiest way to build a CO2 scrubber is to buy some SodaSorb HP (4/8 mesh) and use 2.2 lbs of this absorbent for every 8 hours per person in the boat. You pack enough of this absorbent for your normal dive into canister or any container with a fan and your good to go. If you follow ABS rules, you carry some reserve absorbent to give you an additional 72 hours of bottom time. For ABS rules this this means for say and 8 hour dive time plus the 72 hours reserve, you would need about 22 lbs of Sodasorb HP. As a check, the Deepwork 2000 carries 24 lbs of this absorbent.
The 2.2 lbs for 8 hours comes from http://www.sodasorb.com/English/downloads/Sodasorb_Manual.pdf, page 16 in which is says "In a properly packed and well designed canister... for 8-hour capacity... a canister should hold approximately 1 kilogram of (2.2 pounds) of SodaSorb absorbent".
On my boat, I picked a canister size to meet my normal mission time and then played around with different fan motors until I found a fan size that would circulate the cabin air through the canister without pulling so much current that it would drain my emergency battery bank before I reached the 72 hours limit.
While this worked fine, I wondered about what a "properly packed and well designed canister" meant. I recently came across an excellent technical report that goes into a lot more detail on this subject. It is titled "Design Guidelines for Carbon Dioxide Scrubbers", (May 1983, revised July 1985) by M.L. Nuckols, A. Purer and G.A. Deason of the Naval Systems Center, Panama city Florida. http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA160181&Location=U2&doc=GetTRDoc.pdf
This goes into the impact of design variables such as temperature, pressure, canister length and diameter, relative humidity, CO2 concentration, flow rate through the canister and particle size. The experimental work sited in the paper is for SodaSorb. While this report was written for designing re-breather canisters operating at elevated pressures that a diver would see, it does handle the limiting condition of 1 atm that we would see in a 1-atm. psub.
So if you need a little "light" reading while your doing your business in the morning, you might want to have a read. Now if we could just get someone to drop this into an excel spreadsheet, we would be in business.
Cliff