I have been using Sculpey clay. I was a bit worried the first time I made one, sat it on my deck for the winter. It was in an unprotected spot and lasted the whole winter. In my area we got huge amounts of snow that winter. It came through without a problem. Sculpey is a clay you bake after baking, I paint it then cover it with an outdoor clear coat. The first Sculpey cache has been out almost a year now. The only repairs I have had to do is when the glue I used failed or when a cache has been stolen. I made two of each cache for this reason. I can swap it out and repair as needed without having to really change the container. I mold the glue around a water type container then bake. (make sure container is bakeable) There are other ways to do this without baking the container. But, this post is getting to long, email if you need more information.