Many years ago I made a tumbler for cleaning ammo brass, like yours I used a metal coffee can. The problem I noticed at first was stuff kind of slid to the bottom. So I got to thinking about a clothes dryer and how it uses bars or fins to lift the clothes more towards the top and then it dumps them????
Now I did tumble wet but rather dry, but if you could arrange some inside the drum that would act as fin to lift and dump the media it will work much better. On mine I only fixed one fin because the drum was small and it turns slow enough to not have a balance problem.
What I did on mine, but remember I dry tumbled was I got a sheet of pressed cardboard pallet slip sheet. I folded it so than it had a "V" in it,
Then I cut it to length so that it would fit the depth of the drum,
Then I rolled it up into barrel or tube and cut off the excess or over length so that when I inserted it into the drum it sprung out to properly fit the drum. (I made a liner out of it)
Then when I got it to fit I removed it and glued it back into the drum, bingo!
Now the cardboard will wear out, but not for a long time. I used it for a couple of years until I could arrange my money and bought a factory rock tumbler (vibrating tub type)
Now if you tumble wet the lift bar could be made out of folded heavy wall plastic or other material.
Just look inside a clothes dryer and you'll get all kinds of ideas. (A sidewall paddle is another way of thinking about it)
Mark