The cheapest solar boiler

Step 4

Option 2 you could connect directly from water tank, the water flow By gravity to the shower, the only condition is maintain fill the pipe always.

Future ideas
best way to cath the sun's heat is possible by adding reflectors to concentrate more sunlight to water tank, could also be placing an acrylic or crystal transparent coating to the heat of water tank not escape heat at night, that is doing works like a thermos.