A recent trend in some parts of the country is to put a one-gallon or other large plastic bag (e.g., Ziplocks) around the letterbox—please do not do this. Plastic bags are flimsy by nature—even the freezer bag versions—and will not hold up against sharp sticks, insects and rodents, and the general wear and tear that letterboxes are subjected to in the outdoors. A plastic bag around the letterbox will not last long and needs constant replacement. Even worse, once it has gotten holes, it can fill with water and mud putting the letterbox into an even more hazardous position than with no plastic bags at all.

Additionally, plastic bags do not have the camouflage properties a normal letterbox container usually has, and using them outside of the letterbox will put the box at an increased risk of accidental discovery.

The letterbox container should be the first defense to protect the contents of your letterbox from environmental elements. Containers have proven track records in this regard, but their most common failure is allowing moisture into the box. Plastic bags should be the second layer of defense against this moisture, but they are fragile and need protection from the other environmental elements that the container can provide.

A good general rule of thumb is when in doubt, try a different location. If you find that your box is soaked each time you check on it, then there is a good bet that your hiding location might be too low. No amount of good plastic containers or bags will change whether you are at or below sea level! Try opting for the higher, dry ground and your box will stay dry.