I do not like instructables without a parts list so I will provide one for my instructable.
So this is what you need to have:
1 door stopper (like the one pictured)
A small lever device (to get the stopper off)
1 hacksaw or small wood saw (to give you a bigger space inside the stopper)
At least 1 whole read through the Instructable before you start (so you don't have to abandon the project halfway)
You also need your parents permission so you don't make the house fall down (if that happens you will have nowhere to live!)
So lets get to step #1
Step 1: Step One
Confirm that no one is around otherwise the secret compartment will not be secret and we don't want that do we?
You will need to lever the stopper off (see picture above).
When finished continue to the next step (2)!
Step 2: Step Two
So the next thing to do is put a key (or two) in the stopper and put it back on.
Hopefully it will fit, but If it doesn't fit then you will have to mark where it comes up to and take it off again using your lever.
Then you will have to measure from the mark you made to the wall and add 1mm, (in the example I need to cut 7mm) write your measurement down so you don't forget.
After you have your measurement you will need to cut off that amount. (in the example I need to cut off 7mm)
I did not need to cut anything so I don't have a picture it being cut.
BEFORE YOU CUT YOU NEED TO ASK FOR PERMISSION SO THE HOUSE DOESN'T FALL DOWN!
After you have made your cut you will need to go to step 3
Step 3: Step 3
If it doesn't fit you will need to repeat step 2.
When your stuff fits you will need to clean up and stash your stuff and run!
I am the first person to post a door stopper as a secret compartment so please do NOT copy.
That is all and if you have any questions please ask.