I must first say that I am completely blown away by the pressure-cooker shower -- I forgot the relief valve!
But seriously, what a great design! Do you pressurize it with a bicycle pump or what?
I have a solar shower, which is OK, and I sometimes cheat by heating the water first.
But my favorite system involves two galvanized buckets and a length of rope. One bucket nests in the other, and has lots of small holes in the bottom. Simply fill with water, set near the fire or on the stove until desired temp is reached, then tie the rope to the bail of the inner bucket and throw the other end over a handy tree limb or what-have-you. Lather up, then raise the inner bucket and quickly tie off to rinse. I buy 12-quart buckets at Tractor Supply (about 8 bucks) but most feed-and-grain or farm-supply places should have something similar.
The system is easily and safely portable so ladies and shy types can walk off in the woods or use a shower/potty shelter for privacy.