Safe, secure restraint options for middle back seat?

I have a compact SUV and two toddlers in car seats on both sides of the backseat. I also have a secure dog car seat for my 12 lb terrier, but I always have to swap out one of the toddler seats to use it. Has anyone found a safe dog seat restraint option for the middle seat in the back? It can only be 13 inches wide to fit between the toddler car seats. Fingers crossed! Thanks in advance.