I recently wrote an article in which I discussed the common misconception of the weatherproof mezuzah case. While in recent years some companies claim to manufacture outdoor weatherproof cases, it seems, at least in my experience, that many of these are not as good as we think.
Waterproof cases are normally made of one piece with a screw or plug on the bottom that keeps moisture out. They can be made of metal, plastic, or wood. The metal cases are strong but I find that if exposed to heat ie sunlight, they may become hot and the heat no doubt affects the mezuzah. The plastic ones are good, especially the opaque ones, as they are less affected by heat or sunlight, however they easily develop cracks and can break easily, which renders them no longer waterproof. The wooden ones are great as they don't get hot and most have a waterproof coating / veneer over the paint. This works well initially, but after some time this layer seems to come away and then the wood rots and no longer remains waterproof.
So a really good, solid outdoor mezuzah case remains hard to find. If anyone has any experience with good outdoor mezuzah cases, or ideas how to better protect a mezuzah from the elements, please share.