Stoppin is a clear rubber rockstop, 3″ square, has a concave tacky bottom to form a suction on most clean smooth surfaces.  Also works on carpets.  The top has a bubbled surface so you can easily adjust placement of your endpin.  This is not a good choice if you have an extremely sharp tip as it will eventually puncture and penetrate the rubber.
Stoppin Small is half the size and half the price at $6.00