Cut a piece of plywood and three small ones that can lay on top of it.
Step 1: Sliders
Cut two sliders that can be glued onto the plywood to allow the small boards to slide.
Step 2: Glue the Sliders
Glue the sliders to the top and bottom.
Step 3: Key Hole
Carve a keyhole so your key will fit. Make sure to test it so you know your key fits. You will also want to glue down the middle slider and one of the outside ones. Just because the wont need to be opened.
Step 4: Applying Stickers
Apply stickers onto the numbers or paint them on.
Step 5: Arduino
Hook up a arduino to a light so when you open the sliding door there is a light.
Step 6: Final Product
The final product is a house number that you can hide a key in and nobody will find it. I hope you enjoy trying this. Good luck!