We have a small, cheap, mechanical timer MW we got from Wmart years ago.
Still works like a champ.
Here's how I mounted it. I removed the lid and mounted the lid under the cabinet. I had to add a cross member to the frame of the cabinet for the screws holding the lid to the bottom of the cabinet.
It is a small MW and is 700watts. A bigger MW may weigh too much.
The newer Inverter technology Microwaves are at least 15lbs lighter. That's what I would get if this one breaks. Currently the small package MW's do not have the inverter tech.
Jim