did you read the 2nd post following ? or watch the video ?there...

did you read the 2nd post following ? or watch the video ?
there are dusts or powder as in 'poss off' that are much cheaper, and cannot be tampered with, by the vandals

and the vaseline and vicks vapour rub mix, that you spread around the exterior, I assume it is the smell and getting it on their paws that would deter them.

so we can assume the possums are not the only problem, but human vandals also contribute to your dilemma

having your plot near the tree, that is probably possum's home would not help

maybe you can get the other residents to demand some action
and look at changing your plot to another, away from the tree, and where it may be easier to protect your plot better
cheers
are there any other possibilities ?