Cut a piece of cardboard to fit your frame (or use the back if you're lucky enough to have one).
Cut a piece of fabric to fit your frame. Cut it only a little wider than your frame but almost twice as long as you need it; then fold it up to make a pocket on the front side. Fit it inside your frame and hot glue the raw edges down.
Yay! That 'B' is headed for the metallic treatment.
Can you see the pockets? They're a little disguised. You could even use a contrasting fabric to make the pockets, or add some decorative pom pom trim!
And now to train myself to stick the mail in there.