Have an old broken pocket watch? Have some secret note you need to hide? Do I sound like a commercial?
Here's how I took my 2 dollar pocket watch from china town, and made it into a special compartment.
Step 1: Supplies
Not much really, Just a pocket watch, and a thin prying tool. I used a letter opener. You also may want glue.
Step 2: Pry Off the Back
On the back of the watch, there is a cover, you will need to get this off. It takes some work, use whatever you have, a knife (be careful not to cut yourself), fingernail, whatever you can. This is the hardest step.
Step 3: Taking Out the Innards
Use your prying device to pull off the white plastic piece that holds the clock and battery in. I used the 4 rectangles around the circumference of the plastic. Once you do this, Pull out the clock and face. There will be a thin rod attaching the clock to the top. This is what sets the clock when it's twisted. Snap this rod off towards the top, and pull out the clock. You can use this for something else if its not broken.
You'll notice that once you snap the rod, the knob on the top will fall off, that's okay.
Step 4: Push Out the Glass
Now, from the inside, push hard on the glass (keep this, if you want, its also a lens) to make it fall out. Now you have a nice biggish storage space to keep stuff in!
Step 5: Option - the Knob
Now you have a choice. The knob that has fallen off will work like a key for you. But if you don't think you'll be able to keep track of it, put a bit of glue around bottom of the knob (the part that you spin, NOT the button, and then put it in. This way you'll still be able to use the button to open the compartment.
You can also put one of those small, very strong magnets inside towards the top, so that it's not permanently in there, but it won't fall out.
And now...you're done!
Congratulations, you have an inconspicuous storage container with a lock!