Do you have a dead microwave oven? Yes? Then your wish is about to come true!
Step 1: Remove Unneeded Parts.
[EDITED TO ADD:] Here is an instructable that will explain how to do this without risking electrocution:
Step 2: Decoration (Optional).
Next we traced the outlines of the design directly onto the box and taped off the areas that were to remain white. We applied the first color of spray paint. We repeated this for each spray paint color - we recommend going from light to dark - in our case yellow, red, then black. Be sure to allow enough time between colors for the paint to dry completely. We admit to have jumped the gun a bit on this step ourselves, to some detriment.
When all coats are dry, remove all the tape to reveal the design. The quality of the art will vary depending on your art materials and your skill using them - our results were fair - it's a little rough up close but looks great from the street.
After decorating, we removed the flag from our old crappy mailbox and gorilla glued it onto the microwave.
Step 3: Create a Base.
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.