It seems like you have enough of the green for borders if you add corner stones. Are you trying to only use the fabrics you mentioned or are you open to using something else? If you're looking to finish it in a hurry you could use some of the backing fabric for the cornerstones and then you'd definitely have enough of the green for the borders. I like Bella's idea, though. I think black would look good and would pick up the black in the snoopy fabric.