OK. I would build it the same shape and size as a microwave oven, but I would leave out all the normal electronics.
In the side compartment, in place of the electronics, at the top, I would put a little gas flame, like a Bunsen burner. In the bottom of the side compartment, I would put a treadmill with two strong hamsters. Then, I would install ductwork in the compartment so that when the hamsters turn the wheel, some fan-blades along the rim of the wheel would blow the hot air from the gas flame down and into the big food compartment. This would warm the left-overs WITHOUT using electricity from the wall outlet, and without spraying any hazardous high-power RF energy into the kitchen.