Always buy one more of each lock than you need. You never know what could happen during any given period, and that way you can make sure that you have an extra if anything gets broken!
Make sure you make an answer key for each clue AND write down the lock combinations. You may have it memorized after the breakout box, but it is easy to get confused in the middle of the day.
How about hiding a clue (or having the combination) OUTSIDE of your classroom? Enlist other teachers to let you put something on their door or in a folder right inside the room to get kids running!
Try putting a clue on the back of a $1 puzzle from the dollar store. Not only will the students have to solve the clue, but they will have to put the puzzle together first!