These are fantastic - Raven Concealment TopStop Upper Receiver Cover

I've got 11 or 12 uppers and five lowers. In addition to storage and display, I often transport my ARs broken down in shorter cases. I've got about a half dozen of these covers. These covers are the best things out there for keeping gunk and grime out of the upper and stay snugly in place to prevent the BCG from sliding out the back and falling into the dirt or tumbling out and falling onto the concrete floor. They snap into place on all of my various forged and billet models and are removed easily without tools or pins, just a nice snug fit. I just wish they made covers for lowers and would make them available in other bright colors besides orange (I like the high visibility of "remove before flight" type components) to mix it up a little bit.