This patent-pending paint can holder from Lee Valley fits both the flat rungs of a stepladder and the D-rungs of an extension ladder. It holds either a quart or gallon can steady, and you can easily pick it up and move it to the next set of rungs.
They construct the holder from galvanized steel rod and attach a strong magnet on the top of the holder to hold those ferrous-ferrule brushes while you rest. The can rests on the bottom of the holder and is held in place by a mono-polymer belt that adjusts to the size of the container. The entire holder weighs just 9oz.
You can purchase this contraption for $7.
Paint Can Holder [Lee Valley]