Here's another idea.
I'm thinking basically you'll have to do a custom job on this: measure your works and, say with an exacto knife (a box cutter will do too but an exacto will give you more precision), cut a) some clear acetate sheets and b) all the way through some thick, styrofoamy matte or presentation boards matching the dimensions of your pieces; then, glue thinner poster board matching the dimensions of the matte board behind the cut up matte board. Let it dry. *Poke some small, pencil lead-sized holes (like two per painting) in the poster board behind the 'windows'* Finally, place the paintings in the custom holes you created then slide your acetate sheets over them for some protection and presentation purposes.
Why the holes? So if you need to move stuff around or maybe (!) sell a piece, you take a small stick and just pop your pieces out without having to continually grapple at the edges, prematurely dog-earring or curling them.
Now I've *Never really did this myself*. In my mind, it sounds kinda cool..but then, I did eat cookies and salsa together one time just to weird folks around me out.
It's been years since I saw those people.