Ok, the best way is to put the stones on a sponge cloth;
The stones stick less to it; moreover, it is easy to wet it, and the stone detaches easily. Anyway, it is better to do a few quick iterations to get uniform color (does not take much time).
Moreover, regarding wet tumbling, I found a cheap and efficent way is just to put stones in bowl and stir them with a stick for 10 minutes, put them to dry, and do a final quick stir (3 minutes) to get a smooth surface.
