This is an old, but good, project that I built a few years ago. I retrofitted a commercial fire extinguisher to hold and dispense beer. I brew my own beer at home, so it is fitting to have a homemade "keg" to hold homemade beer!
The extinguisher tank is stainless steel, and fairly easy to clean. The top unscrews, and leaves a 3" dia hole. The extinguisher was originally setup to dispense fluid (ie it has a downtube that connects the outlet to the bottom area of the tank, and a port on the top for adding pressurized gas). This is exactly what we want to dispense beer. The main modifications that I made were:
Replacing the original (nasty) downtube with a new clean one
Replacing all of the rubber seals with new ones
Lots of cleaning
Added a low pressure gauge and compressed air quick-connect system (not shown in the photos)
I got my CO2 cylinder and regulator at a local welding supply store. It was expensive, and I am not sure if it was worth it.
In order to carbonate the freshly brewed beer, I keep the extinguisher in the refrigerator with the CO2 tank set for about 12 psi. I give the beer a vigorous shake once or twice a day, and after a few days or a week, it is ready.
The extinguisher is always a hit at parties, and has become a tradition.