Can't you do it inductorlessly if the load is a heater, and you can freely choose the resistance, just by PWMing the element from a capacitor?

If so that's probably like a $3 board in quantity, which could also give you the option to use a USB-C solar panel, which is probably the most versatile and convenient for a lot of people, with LEDs to indicate power and help you aim.

It would go against the low tech concept, but if I was going to build something like this I would probably go for MPPT