I was looking for different ideas for building a digester and found this .pdf file. It is old, but the information is good.
http://c.ymcdn.com/sites/www.echocommunity.org/resource/collection/C01889C5-4BB2-424C-962F-F5B14EF26E69/Biogas.08.pdf
I was considering the sliding barrel method, but now I am thinking about using a sealed barrel with an inlet and outlet and having the gas going to inner-tubes or bladders to supply the pressure. The biggest issue with biogas is keeping out the oxygen, so using a bladder with a sealed barrel
should make the process easier/safer.