The Mopeka Halo is a thermoplastic rubber foot ring for standard cylinders:  20, 30, and 40 lb (5, 7.5, and 10 gal) DOT cylinders

Protects your car and floor surfaces from the rust and grim that builds up on cylinders.

Halo also provides the benefit of providing the space and improved wireless range when used with a standard sensor.  Great replacement for lost aluminum spacers.