I usually go with the light switch/outlet screw but you said those are covered. There must be grounded metal somewhere in your house. The microwave body, a pipe or faucet, you could even get one of those grounding wrist straps that plug into the ground port on your wall socket.
If it’s attached to the wall theres probably bare metal on the bottom, and if not there’s definitely some on the inside when you open the door. Just any bare metal on it.