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.

https://www.hammfg.com/ https://www.budind.com/ http://takachi-enclosure.com/ https://www.adafruit.com/product/903