How about a few light sprays of ClearCote, (KleerCote?) not sure of the spelling, it's used as a hobby sealant/protective hard coating and comes in matt and gloss.
I've used it on plastic models before after painting/decals etc, not sure how it would react long term with paper though, too much would obviously cause sagging!
I've used it on plastic models before after painting/decals etc, not sure how it would react long term with paper though, too much would obviously cause sagging!