Would some of the larger Pelican or Otter Boxes work? They seem to be very strong, waterproof, and lockable. They come in a variety of sizes so you could probably get exactly what you want as far as volume goes. I think you can even get the Otter Boxes in a very bright yellow or orange color designed for high visibility. This would save you the time, money, and energy of having to paint it yourself. I'm sure there are other possibilities but those were the first to come to mind.