Willib,
To my way of thinking, it all begins with finding a consistent feedstock, the type of feedstock will help dictate the type of digester, and the amount of feedstock will dictate the volume of methane you're capable of producing. I kind of work this backwards because just about any bio-mass will digest and produce methane given time and the proper circumstances, but some bio-mass is better suited to one digester type or the other.
I like testing with batches, so I can better track and log results, but to me it seems a production digester would be partly dependent on testing results so my primary focus has been on testing different feedstock knowing that there are "tried and true" digester designs out there.
That 5L (1.3Gallon) beer keg idea looks ok, the DIY fittings are more interesting, I would want to test in 1 or 5 gallon pales in that smaller range for their removable tops. I'm real interested in organic compost, so for me, methane has an interest as a by-product, who knows maybe there's more there, but for me it doesn't really matter.
Curbie