Yeah, I'd probably use some good sized bolts with the nuts underneath the car, odds are nobody's gonna be going under your trunk undoing those nuts. It wouldn't hurt to put some Liquid Nails or silicon on the bolt on the inside of the box to make sure there aren't any leaks, but other than that I don't see any problem with it.
You could also go with the way my car ended up, make the box so big and heavy that it won't move an inch and you're not getting it out unless you know exactly how to work it out of there. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif