I recycled some cereal/cracker boxes for this project. I cut them in rectangles 3 x 2 1/8 in.
On most of my cards I wound one spool of ribbon, but if it was extra thin or didn't have much left on it I fit two or more to one card (pictured below).
Reminds me of my DMC boxes with all my DMC thread organized just like this! I put mine in shoe box lids so as to keep them open and accessible and because they wouldn't fit in my small drawers -- the thick ones were too tall!
Thanks 'Sophia' for suggesting this "new" method!