Cap seals come in a couple of different styles for your different needs. You may want to provide tamper evidence so you know if your product has been accessed. If so, you will want to get a crimp-on cap seal and the tool that goes with it. If you don't need that much protection, but want to cover the bung from the elements or dust, you may just want a snap-on cap seal.
Perhaps you need a little extra security. A drum seal or lock may be your answer. These drum locks provide security on location or while the drum is in transport, and they are safe, simple and easy to install. They are available in different styles to fit your different situations.
Once your cap seals have been installed, they will take some work to get off. You might want to look into a Quick Cap Seal Removal Tool to make your removal a little easier.