This kettle will heat 16 oz of water in full sun to a nice hot/warm temperature for some sun tea or washing hands, but you'll have to wait a few hours or less depending on outside temperatures. I've only recently made this during the winter and have achieved 95 degree water temps during 30 degree outside temps. Someone in a warm area of the world will have to make one and tell me how hot it can get during summer-like temperatures.
(Instructables member Kopomeroy built a few versions of this; they achieve 140-160 F during 80 F outside temps. See comments below.)
This takes three plastic bottles to build, one 2-liter, one 16 oz, and one 20 oz. Also needed are flat black spray paint, 5/8" (16mm) rubber heater hose, aluminum foil tape. The outer 2-liter bottle should preferably be clear.