I pieced the back of the orange quilt together and measured it out. I had two inches total to spare in the up and down direction. Hmm...that doesn't seem like enough. But...I didn't have enough fabric to make it any bigger. So I decided to just try it and deal with it if (perhaps when) it didn't work.
I put it right up to the edge on the top. I ended up with about a 1/4" extra. (The shadow on the bottom is the extra.)
That meant I had about 2" left on the bottom. It fit!