I think either idea could work! Maybe zip ties for the upper strands that won’t have to travel as far to avoid the tunnel. Then the jumpers or mending kit for the lower strands that will have to travel a greater distance from one side to the other.They make quick connect cables in 30 and 48 inches meant for connecting their fences and they're only like 6 bucks a pop and i bought 2 just in case so thats not a bad idea! From my understanding as long as the fence is only touching itself and not the ground or tunnel i shouldn't have any voltage problems so I was thinking of ziptieing the lower strands together to scrunch it up just over the tunnel something like this (the red are zip ties) insyead of cutting it as i worry itll unravel but i like your idea of the fiberglass posts and using the mend kit better i think!
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Edit to add: you don’t want to have the strands scrunched up and lifted away from the base of the tunnel too much or you are just inviting smaller predators in after all.
