Ideas for filling the bottom of a fence gap for a dog?

I have a neighboring fence with a 2”-6” gap at the base on the backside of my yard. I’m installing a fence for my dog and need an idea on how to fix this. The fence isn’t mine, and they won’t pay to fix it, or let me fix it. He’s a big dog, 70+ lab mix. He’s not outside long by himself, but I don’t want him getting underneath quickly and taking off if he sees something. It stretches about 60 feet across the back with that varying gap. I don’t want to double up the fence, that might look a bit dumb. Ideas?