I was a bit sceptical that it would work, but I invested £1 on a pack of 100 cable ties (295mm x 4.8mm) and tried it out. My scope is the Leica 62mm (angled) and I wrapped the cable-tie over the stay-on case. After calibrating it as described I just had 15mm to cut off. I've since found that bending the excess rather than cutting it off allows for easier sighting. I always carry my scope in a rucksack and thought this might affect the cable-tie. It doesn't really move that much and can be very quickly moved back into position.
Anyway, it really does work and makes locating birds almost guaranteed.
I now have 98 cable ties for sale if anyone is interested.