Ok, here's my 15 sec brainstorm: Coat inside of mixer body with bedliner or whatever so cases won't ding against the sides when all the media is gone. Drill a bunch of holes in the main body of the mixer, big enough for the media to flow out, but small enough to keep the cases inside. Rig a cover to go around the mixer body for when you don't want the media to leak out. When you are done cleaning the cases, put a big tote or basin underneath, pull the cover of, and start the mixer turning again.
Like I said, that's only 15 seconds of thought....
