If the box is plastic, you could avoid making any holes by using a reed switch inside the box and a magnet outside it. Some sort of a permanent clip for the magnet might be needed, depending on the operating mode you envisage & the duty cycle. If the circuit takes more than the 50mA or so a reed switch will cope with you will need a secondary switch, e.g. a MOSFET as well.
Incidentally, beware of waterproof plastic boxes. In my experience, when exposed to the great outdoors, the combination of sun/UV and temperature cycling will warp the plastic enough to relieve the compression on the gasket seal, which
will then leak. I think you need a screw fixing at least every 8cm or so to keep things tight. Or do what I did, get rid of the gasket, and replace it with outdoor flexible 'decorator's caulk' (PVA-based). Silicone works as well, but is very difficult to get past when the time comes to replace the batteries. I speak from experience