You are going to invent a new device that people can use instead of a microwave to warm up leftovers.

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.