The way the microwave was mounted in my boat was screws came up through drilled holes in the cabinet floor and went into the bottom of the microwave where its feet were mounted. Plenty secure enough for a sea going boat.
In my Oliver
, I replaced the existing microwave and mounted the new one by drilling through the back wall of the oven area and running three screws into the wall behind it. Just pushed into place and ran the screws through the back all. That area of the oven was only a single thickness of metal with no electronic or moving parts. Behind it, the back of the cabinet, is 1/2" plywood.
You have to make sure the door operates when it is in the position you want it and it may require some spacer blocks behind or underneath. See how the cord is managed to make sure it doesn't get pinched.