The only time I have ever lost siphon was when I was messing around with mine. In your case, it would probably be harder to start the siphon since you wont have much acess to the space in the back of the box to run tubing to start it. If you decide the box is your only route, you could take the u shaped tube and put a hole in the very top with a valve. Attach airline to it, open it, and start your siphon via suction. I would seriously consider drilling in your case.

The other thing to remember is ventilation as you do not want soggy walls.