don't know if i can help roger but here goes,
usualy when you put a possom collar around a tree it needs to cover
from about 1.5meters from the bottom of the collar, usually collars
are made from falt sheet tin app' 18 to 20"s wide.
using there tails as another arm they can reach about the length of
their bodies at that point. don't know if they can climb brick walls
but wouldn't doubt that they can not much stands in their way.
when you collar a tree yo must make sure that there is no other trees
or high fences anywhere near the branches of that tree or the collar
won't work they will use the other route.
'it works for me it could work for you,'
"in the end ya' gotta do what ya' gotta do" but consider others and the environment