I used at 5-1/2 X 7-1/2 inch piece of cardstock, scored at 1" along both long edges and at 3-3/4 and 4-1/2 along the short edge, then folded it into the box sleeve. I used the 1" circle punch to punch out the thumb hole.
I made four coordinating 3X3 cards and envelopes that slide neatly inside.