The backing on the Forma or any device isn't the top priority for me when making a purchase. For others, my recommendation is a custom fit skin (link below). Smooth, matte, solid colors or thousands of designs, one only needs to apply it to the back. It is easily removed and causes no damage and leaves no residue on the device. This company produces high quality products with excellent customer service:
https://www.decalgirl.com/skins/kobo-forma-skins