This is the template for the card keeper I made for my daughter. It is a box that an audio book came in. My scanner wasn't big enough to scan the whole thing, but the other side that is missing is exactly like it's counterpart.
It could be made out of posterboard or cardstock. The 7.25 inch side could be cut down to 6 or 6.25 inches and still work for a standard quarter sheet card. The one I made held 9 cards with envelopes and ribbon embellishments (but it was tight), so keep that in mind for the width.