2 things:
1) "Dog Proof" Coon traps - look 'em up on Youtube
2) Add 2x4 welded wire over the poultry netting. TSC sells a 100' roll, 4 ft high for about $70. Run that around the pen. They might climb up past it and try to get in, but you just made life a whole lot tougher for them. 1/2 HW cloth...