Rather then try to "fix" the back, you could use clear contact-type paper like this to give it a glossy finish (if you're good with an x-acto blade):

https://www.amazon.com/Adhesive-Covering-Removable-Protective-Antifouling/dp/B07PLX43LP?th=1

If it looks weird, try hitting with a hair dryer. If it doesn't work, not much lost.