At Principia College during the sustainability conference we built a functioning mini-digester out of paint buckets. It really works and not only teaches the principles but gives a few minutes of daily gas!
The participants in the workshop built by watching the following videos (and heck, if they can do it, so can you!)
The first video is an animation done in Blender 3D:
And here Culhane's students at Mercy College NY show the system they built.