I was using an ammo can but everything was just crammed in there. I'm not convinced it needs to be 100% water tight either. The drier the better obviously. Yesterday I started making a new box for mine. I made a small aluminum pan with a treated 2x6 in the bottom. The wood allows me a spot to screw the MS box to and isolates it from the frame. I was having some electric noise problems so the more isolated everything is the better for my car. I have the relays on the side and the Spal fan controller velcro'd to the top of the MS box. I plan to run rubber weather stripping around the outside edge and make a lid. I built it with the two tabs so I can just put a bungee of some sort over top to hold it down tight but allow easy access for the tuning cable or to fix all the crap that seems to go wrong. No lid yet but here is the base. Still need to silicone the corners, if not weld to seal it a little better. My car is kept in a garage and doesn't sit out in the weather.
Here is what I had before. It worked well but I couldn't get to anything to work on it without taking it all out. The tuning cable was crammed in there too. It was easy to built though.