Very, very cute!!!
I would do plain sashing in a yellow, I think. Might be a little too bright, but something like this:
https://www.fabric.com/buy/0314234/k...dog-word-black
Maybe with a border of dog bones or something similar:
https://www.fabric.com/buy/0314231/k...og-bones-brown
https://www.fabric.com/buy/0314235/k...dog-word-cream
https://www.fabric.com/buy/0314234/k...dog-word-black
Maybe pick the border fabric first, then a coordinating sashing fabric.