I wouldn't bother putting a sheet in it. If you are using flannel for the back, it will be plenty heavy for a summer quilt. I have a summer nap quilt made by my great grandmother that is pieces of house-dresses (frocks), sewed onto muslin squares (flour sacks), some hideous cheeto-orange fabric added for wide sashing strips and an old table cloth for the backing. She tied it with 2 strands of embroidery floss and used a knife edge binding--tucked in the back & front and top-stitched it all around.