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Re reading your post it sounds like you only want to discourage the dog while on possums. Would a bark collar be enough gun? You could always try the 2nd hand market for something like an Alpha but...

Why not go for an alpha 100 unit, and TT15 collar, plus side is you get a gps for free!
You can get the dog to recall to the beeper sound that the collar makes, I have seen this with my own dogs.