It must be smaller than 55cm and be able to heat up a half gallon of water
I have an idea how to do it. You need 1m of PVC hose(3cm in dia, black), three 2L soda bottles(clear), alum foil tape, a couple of clampers. Make holes in bottoms of the bottles(aprox.3cm), cut off the bottles necks to have holes of the same dia. Put the hose through the bottles. Stick alum foil tape to the cylindrical parts of the bottles. Clamp one end of the hose and pour the water, clapm the other end. Leave it in the sun. You may tilt the construction a little for a better result. Being on a hike you can disassemble it easily and use bottles as containers. If the weather is being favorable in your place and you have time to do an experiment, I woudl be grateful for posted results because I want to built this construction on a larger scale.
You could make the solar hot water kettle with and 2-liter and a 3-liter plastic bottles. It's a basically a mini solar batch heater and will heat water to 95 F - 160 F in about 2-3 hours depending on outside temps.
Camping and/or Outdoor stores may have camp shower bags (thick, heavy-duty, and black; holds anywhere from 1/2 to 5 gallons, simply hang in the sun). An on-line store would have them as well. I saw one many years ago in a J. C. Whitney catalog. Search on the internet for 'camp shower'. HTH