Mmm. Somehow I think the Solid Signal guy is clueless. From what I've seen of chimney mounts (not Ronard) the strap length is the maximum length. The strap folds back over the brackets and is held with a clip. You cut off the excess.
Maybe try buying from the Channelmaster store? They sell chimney mounts too.
http://www.channelmasterstore.com/TV..._p/cm-3080.htm