Seeing that reminds me of my invention 30 years ago of the self-forming sealed solar trough - a spin off from our research into large cheap flexible mirrors for astronomy. Circular ones failed cos we could never get precisely circular supporting rings. It was instantly obvious whether they were made on a 3 or 4 jaw chuck by the error distribution from a point source. But quite acceptable for focussing IR onto some collector
No need for precision formed mirrors, let the heat of the sun expand air in a glass topped box/trough against a flexible mylar membrane - ie crisp poke material and run a copper pipe or similar up the centre. Put it on a slight incline, point it vaguely at the sun, cold water in one end, warmer water out the other.