Because your center is applique, I would do an applique border. A simple leaf and vine with some flowers in the deep red would be cute. Or you could search Google images of primitive angel applique and maybe get some ideas from that.
If you do it 6" wide that would increase your size to 54" x 58". That is still on the small side for a throw so you could frame it in that red gingham by putting 1" on either side. This would make it 58" x 60" which I feel is a nice throw size big enough to nap under comfortably.