An MPPT would double the cost of this setup.

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

You've gotta count the cost of your time as negative for that to be true. The author built multiple ovens from scratch here, and every cooked meal could be done sooner if the temperature were higher.