Dycem Cello Black Hole Cello Mat
Non-slip cello mat with superior gripping properties prevents sliding on just about any surface
This works really well as long as the floor is clean. It is small, compact, and very sticky. Unfortunately most of the places I play have dusty or dirty floors, then it won't hold at all. So... I'm back to a grommet belt.
FYI the center cup is made of plastic, which I prefer to metal.