My DSL modem is in a tupperware container screwed to the wall outside on the porch. So that's an idea. Just about any kind of box could work. Purpose-made enclosures for routers are available (just search Amazon), but they are overpriced IMHO.
I am a lawyer, but not your lawyer. And who cares anyways? We're here to talk construction. This is DIY advice, not legal advice.