I also have this charming alphabet print flannel I can use for the back!
If I do three columns of five blocks each I have six inches left over for separation strips and/or borders. I've been auditioning prints... This one struck me as too busy. Hmm... It looks better in the photo than it did in person.
The art nouveau print seemed too busy again and the cream was not strong enough to be on the outside edges. Not that I would have sewn it up the way you see it here. At this point I was just trying out various colors and prints in general. Those strips would all finish at 1.5".
In this layout I have the art nouveau stips cut to finish at 2" and the cream print to finish at only one inch. On my design wall I like this version the best.