Although some will say this thread is useless without pictures, I'm certain people with these clamps will be able to place 2 -2 together by way of description and get this sorted.
I have been wrestling with the slipping issues on these clamps for sometime now and just had not sat down to work it out.
I found rubber similar to this...
http://www.amazon.com/Butyl-Rubber-Shee ... B000MAL0K2Although mine is somewhat thicker and more sponge based, I used contact cement to glue this to both sides of the clamp. The C (curved side) and then against the flat (Battery) side, trimmed after drying with a new razor knife blade.
Since this application I've had ZERO slippage either high or low mounted on the stands.
I have tested this in -10 to 85 degree weather and I can claim it to be a usable, viable and permanent fix for this issue.