In this Instructable I am going to show you, how to make a Lego storage box, that can only be opened if you know the trick. Ever since I have seen this official LEGO Storage Brick I fell in love with it. The only thing that really bothered me was, that they slide open when you try to lift them (and the cut-out at the sides that is supposed to make opening it easier...).
I remembered this Mystery Box by fungus amungus and decided to use his concept, to make a better Lego storage box.

If you don't mind the cut-outs on the sides and the uneven bottom, you can of course add the lock to the original Lego storage box.

Step 1: Stuff You Need

Materials

6.5 mm plywood
1.5 mm wood (as spacers)
8 cm wooden pole
paint, primer, paint brush
wood filler
M2.5x16 mm screws
M2.5x8 mm screws
wood glue
magnetic lock (e.g. on amazon)

Tools

angle clamp
table saw
drill
80 mm hole saw
2 mm drill
7 mm drill
sander, sanding paper
clamps

Step 2: Attach the Side Panels

Start by attaching the side panels to each other, as shown in the first picture. To prevent the panels from breaking, drill 2 mm holes into the wood before screwing the screws in. An angle clamp is certainly very helpful and wood glue adds some extra stability.

Step 3: Drilling the Holes

Next you will have to drill the 80 mm holes. You can find the measurements in the first step. Afterwards you can use the plate as a stencil to mark where the wooden disks go later. Simply align the diagonals, as shown in the last three pictures of this step.

Step 4: Assemble the Inside Box

The inside of the box consists of two parts. One with the holes in it, which has to be attached to the smaller side panels and one which will be the actual bottom of the box. Attach it to the higher side panels and glue both boxes together, as shown in the pictures.

Make sure, that all the screws are screwed deep enough into the wood, so that they can later be hidden.

Step 5: Assembly

Make sure everything fits and cut the 1.5 mm wood to fit inside the lid of the box. It will reduce the friction and hold the inside perfectly in place.
Since it is easier to attach the lock before the top plate is screwed to the sides I suggest to do so at this point (even though you will have to unscrew it later before painting the box). You will have to mount it to a small wooden plate and drill a 7 mm hole in it, to fit the magnet (as shown in the second picture).

Use the table saw to cut the wooden disks to a height of 2 cm and use wood glue and screws to hold them in place.

Step 6: Fill, Prime, Paint

Cover all the screw heads and possible holes with wood filler. Let it set and sand all the pieces. Use a few layers of primer and sand in between painting, to get a nice and smooth result.
Make sure to paint in thin layers, since there is not much clearance in between the boxes.

Congratulation, you are done.