I personally use a magnet probe holder, I previously used suction cups but they don't last.
SoCal Tropical Fish Outlet is another good source of probe holders (and float switch brackets too). The photos on their website are terrible, but the products are good at reasonable cost. I'm told that they have them custom made by a local plastics shop.
http://socaltropicalfishoutlet.com
That looks nice and cheap, but my sump level isn't nearly high enough for that to work with my setup.
Sorry just re-read the ebay post looks like that is how its intended to be used.