If you have a Chicago brand 3lb Rotary Rock tumbler you can use this insert to make the tumble space a little smaller and add some agitation. Recommended at any settings for dry medium. For wet medium it's recomended to print at 100% infill and/or seal the inside with a 2 part epoxy resin before using.

If you don't have this same rock tumbler you can see how easy it is to make one here: https://youtu.be/xVJ-5kUvYMM