All it takes is a water pipe painted black zigzagging inside a box which is black inside and has the sun facing side replaced by glass.

You can get hot water from something like that even in Winter.

we do. they are about the size of a solar panel, look similar and are on some roofs of houses in my area. the ones i'm refering to are not for boiling water, just for preheating so need less energy when it goes back into the [ger: heizungskreislauf]. all the ones i know are older then the current PV panel boom. fun fact, there are even concepts (and implentation, but i can't find it) for vacuum glas tubes that have a parabolic mirror around a black painted copper pipe, which should be able to generate ~400 °C "process heat". but they are not wide spread, cause PV is so much cheaper now