Well, there is a hard plastic holder made that is credit/business card sized. I have seen them used to hold security cards or a few business cards. A slot that's slightly wider than the holder and not deep would hold it there for display.

For glass, honestly, I'd go to a glass shop and see if they could do something in a museum glass. Front and back sandwich kind of thing and then a less loose slot than described above.

Working on a frame of that size would drive my fat fingers to revolt, so maybe a couple pieces with a dado coming out vertically from the base so the glass sandwich could slide in... but I'd cut those dados on a larger board and then rip off the part to be cut down into equal lengths. The base would just need a couple square dowel holes. So a good reason to strop my chisel there.