I also saw this at a demo. The size of the cardstock is 8 1/2 x 3.
Score at the 1 3/4 point and again at the 2 1/4. Then score again lower at the 5 and 5 1/2 point. Fold on the score lines, the shorter end will be the top flap. Velcro would work, also you could set 2 eyelets .One on the upper flap near the bottom edge and one 2'' down from the top of the bottom flap. Thread a cord through the bottom eyelet, tie knot at back. Then to close pull one tread through upper eyelet and tie!!
Hope you find this useful.