I have got good results by pouring water into a dark garbage bag, enough for a military shower but so it lays flat and only 2" deep. laid out onto flat ground, leaves underneath if the ground is cold, around noon the bag collects a considerable amount of heat.

Once warmed sufficiently you can snip a corner and hang the bag, actually hang first then snip if you go this route. Or you can pour the water into a bucket with holes.

If your doing it a lot it pays to set up 2by4s on edge into a square and rig, glass, clear fiberglass corrugation, or plastic over top. The key hear is to surround the bag and place the clear cover in such a way that it maintains an inch and some of airspace in such a way that the air space is pretty well sealed.

We connected four lengths of 2by4 with hinges and used Plexiglas that we drilled on the corners so lag bolts and wingnuts held the cover on tightly. We sized the frame to accept a standard twenty gallon garbage bag and placed the frame and bag on top of a piece of iso-cyanurate insulation to keep the warming water off the colder ground.

As I remember it a batch went from about 45F to something around 130F, a warm (not quite hot) shower, in twenty minutes at noon. With two setups being run we showered eight people in about two hours. A few two at a time. Given none had had a shower in two weeks it was rave reviews all round. YMMV.