This is a simple yet very clever design made by killrwulf that I found while looking through the secret doors and compartment contest. It was the first instructable I ever made, and I made it because of its simplicity.
Step 1: Materials and Equiptment
-one card deck (preferably not a good one)
-a pair of scissors
-a glue stick
-two rubber bands
-a stanley or craft knife
Step 2: Cutting
First remove any six cards from the deck and place them to one side, they won't be used until later. Next an outline needs to be drawn just inside the edge of the card to show where to cut (in my case a line was already there). Then stab the stanley or craft knife through the card and, using the scissors, cut out the inside of the card. Repeat with the other cards, excluding the six that were placed to the side.
Step 3: Gluing
Place a sheet of paper down so glue doesn't get onto your workplace. Then put glue on the top of a card and place another card on top. Repeat this process until it looks like a hollow card deck.
Step 4: More Gluing and Assembly
Split the six left over cards into two groups of three and glue each group together so there are two three card thick groups. Bend one of these groups in half (I strengthened it on the back with sticky tape) and then glue the not bent group onto the bottom of the hollow deck.
Step 5: Optional
I wanted to make this hidden compartment look good as well as being functional, so I glued thin cardboard around the inside of the hollow part so I didn't see the different layers of the playing cards (I just used trial and error to get the right sizes for the cardboard). Next I painted the inside white to make it easier to see whatever I put inside it. Note: be careful not to paint on the outside of the card deck, otherwise it gives away the secret part of the compartment.
Step 6: Beginning of the End
At one end of the card deck mark where the glue will go when the lid or folded group of three is added. Glue only one half of the lid otherwise the lid won't open. once the lid was glued down I decided to add a rubber band at the glued end to make it look authentic. to do this I drew a line around the deck of cards, put glue on that line and added the rubber band (the glued down rubber band prevents you from trying to open it at the wrong end.
Step 7: Finishing Touches
Add another rubber band to the opening end of the lid for less conspicuousness then hide whatever you can fit into your secret card deck.