Cut a piece of scrap wood the size of the bottom of the mailbox. Drill two holes in the bottom, and two corresponding holes in the bottom of the microwave.
Use these holes to mount bolts that will hold the box in place. The bolts do not permanently attach the microwave to the board. It simply sits on top and the bolts keep it from moving about. We ourselves are not overly concerned about microwave mailbox theft, but if you are we might suggest using some industrial glue or cement at the base of the bolts where the oven will be at rest.
For our box, one bolt is very long and the other short - the longer extends into the space where the machinery once was and the shorter is on in on the side where the inside of the box is. It was not necessary to drill the hole all the way through into the box space in order to have a bolt long enough to secure the box onto the scrap board.