I've never heard of using any of the things you mentioned, but I think something like a clear furniture finish (polyurethane-acrylic) might work? Had to look up mod podge- yes, I'd try that. :gj: I have done decoupage before, & that's a similar product to what's used to finish furniture.
If they're left uncoated or unprotected in some way, do be aware that some types of moths WILL eat them. Ask me how I know...;) And mine were in a plastic bag, but just not sealed as well as I thought. So apparently you've been lucky so far.