Anyway, to finish the edge you can either fold over the back piece and hem
it down (if it is large enough) or you can add a separate binding strip,
folded in and hemmed down front and back. 2" is a good width to start
with, since that will give you two 1/2" folded hems, and a half-inch-wide
binding to fit over the space you have left. You can purchase something
like bias tape, but it might be nicer to make your own from the same
fabric you used as the backing.