I did say that I would install a forced air circulation system that includes heat and air conditioning, not just a "breathing hole", and the dimensions of the gear tunnel are quite different than those of a trunk (I also said that the dogs would need to be able to both stand up and turn around)...
Given that it looks like there's a passthrough from the cab to the gear tunnel, a crazy idea popped into my head. I'm wondering if it would be feasible to leave the passthrough open, use a 12v tube fan with small air ducts/hoses run to each end of the gear tunnel for air circulation and...