You may have solved this by now, but I'm wondering... how much clear space is there inside the box, where for instance a drill bit would go in when drilling out the hole? If it's an inch, great; if it's 1/4 inch, you'll have to be pretty crafty if you choose to drill out the hole.

We don't know your location, so it's hard to tell if something freeze-proof is needed. That might limit what you can use.

Anyway, I'd suggest a neoprene washer, larger than the hole, on the outside of the box, held in place inside and out by a fender washer. The screw or bolt that holds this tightly together should be a nylok lock nut.