There are a lot of companies that make electronics enclosures. Some of them are waterproof. Usually they have a couple pre-drilled mounting flanges to screw in the board as well, and then you'd drill your own external holes for input/output as necessary. Here's a couple manufacturers, and you can usually find stuff on e.g. adafruit and digikey as well to save on shipping if you're buying dev boards or other parts from them already.