I was out scouting locations for a cache I intend to hide. As I drove by I spotted this place. At first I thought it would be the perfect location for the cache I was planning to hide, but after more consideration I decided I needed to keep looking. Despite that, I still thought this was an interesting out of the way location for a hide. Back to my geo-car, I looked through other cache containers I had on me.... Ah... this one. This one would work well here.
You are searching for the world's smallest ammo container. Small enough to be an ammo container that Ant-Man might use (if you know that reference.) The ammo container is not large enough to hold a writing utensil... so bring your own. Happy hunting.