I've been searching for it, and I can't find it, but someone made a shower similar to eamarquardt's pressure cooker shower using a refillable fire extinguisher.
The gist of the article was that you fill the extinguisher with water and put it on the campfire without the lid on it. When the water got to the desired temp, he took it off the fire and put the lid on. He then pumped the extinguisher up with a tire pump (although he mentioned that using a 12V tire inflator made things easier) and off to the shower he went. I don't remember exactly how much pressure he put in the extinguisher, but if he used a tire pump, it couldn't have been much.
In my search, I did find this video. This guy did a nice job of modding the extinguisher to make for a good looking shower.
The only difference I can see is that this modded extinguisher wouldn't go on the fire. You'd have to heat the water separately, then pump it up in place.
Having said all this, I'm keeping my eye open for a cheap refillable fire extinguisher OR a cheap pressure cooker. I think both are great options.