First of all, what is the size and weight of your possums?
when snaring animals on running poles or leaning poles if you use leaning poles, the thinnner the pole the animal will hold onto the sides of the pole when climbing and hsi head will be lower to the pole. If the pole is thicker, E.g 4 inches in Diameter, the animal will run along the top of the pole therefore it will run higher. To snare grey squirrels a 4 inche noose , 4.5 inches high. The same goes for Pine Marten. When snaring in Canada. The running poles are not illegal in the UK.
I tried to send you a message earlier this evenin