Turn off the electricity in the room where you will be working. This will eliminate the risk of cutting into a live line while you're sawing into the wall.
Measure the the top of a metal candy or mint box. Use a tin with a tight-fitting lid. It should also be smaller than your outlet cover.
Mark these dimensions on the wall where you will place the jewelry case. Choose a spot that's logical for an outlet but out of the way, such as near the baseboard halfway behind the headboard of your bed.
Cut out the shape of the candy tin with a keyhole saw. This small saw will allow you to cut tight corners and small shapes for the best fit.
Insert the plug piece into the outlet cover, and glue them together. This will create a complete outlet face.
Center the candy tin on the backside of the outlet plate and plug. Glue the tin to the plate, with the cover facing outward. Allow the glue to set and dry.
Place the outlet cover against the wall, pushing the tin through the hole in the wall. Adjust the size of the hole by shaving the sides until the tin slides easily through the hole without scraping the sides. Remove the outlet and tin.
Fill the tin with jewelry, money or any other objects you wish to hide.
Pinch off four pea-sized pieces of poster tack. Stick one piece on each corner of the backside of the outlet cover. Replace the cover with the tin sticking through the hole into the wall interior. Push down firmly on all four corners to ensure that the cover sticks to the wall and looks natural.