metal drums offer a big “find” profile for metal detectors and big brother’s flir in planes and choppers. wraps of ratwire suffice to keep out rodents from plastic drums. Also, metal drums rust/corrode very swiftly in many areas, despite your having used the “french drain” of gravel around and under them. A mere 2 years and most metal drums, in most areas will be breeched. if you don’t first paint them with rustoleum and don’t bother with the gravel, some will leak within a year.