I used modge podge instead of matte medium. Quite different. I see pro's and con's with both.
Modge podge: Very thick (like thick school glue) smells like glue tacky and dries quickly less expensive I got fewer wrinkles and air bubbles in my collage elements (yay!) slight sheen to surface when dry
matte medium thinner not tacky, you can reposition collage elements matte finish
I will probably switch between both in the future depending on what effects I want. It's good to have choices.