Follow the Adafruit learning guide to print your circuit board using the following Eagle CAD files. They are located in the eagle sub directory. Or just grab it from the github repository
git clone https://github.com/shellbit/mailbox
A printed circuit board isnt required but it does make the project a little neater, wiring-wise
Use a coping saw to cut the board to size so it fits inside the small plastic enclosure box. I placed the board between two thin pieces of scrap wood in a vise to avoid damaging the board while cutting.
NOTE: The board is sitting upside down in this picture.
