Since the ribbon I ordered was all wrong, I decided to try a 1/8 inch red ribbon as a border for the spine of the cover and I believe it worked brilliantly.
Here's a closeup.
I don't think I will add anything more for the spine. I'm thinking about trying it for the front of the cover, accented with a couple rows of embroidery. Either the red ribbon with purple and gold embroidery or purple ribbon with red and gold embroidery. That's if I could find some purple ribbon in the right width and color.
And I still need to do the red embroidery around the edge of the back of the cover. It's to cover any raw edges like at the corners and to give it a more finished look that is consistent. It would be narrow like 1 or 2 rows of a stem stitch, same as I would do on the front.