Rock Tumbler chamber

My Harbor Freight rock tumbler seems to be breaking down -- the big rubber chamber / cub is leaving black rubber residue on my parts. The rest of it is fine, so I made this chamber to fit inside the existing rubber thing. The lid is VERY secure, and will need to be pried off. I could probably add a chamfer or fillet to make this easier, but the design-as-printed works well for me.

I added ridges to pull material toward the bottom, which hopefully helps things mix better.

The lid isn't m