Cardboard (cardboard with white on one side works best)
1 Ziploc Storage Bag
Dry Erase Markers
Step 1: Cut Your Cardboard
Mark the dimensions 8.5" x 11" -the size of normal printer paper.
After cutting your cardboard with an Xacto Knife or similar cardboard cutter, if you are using cardboard that's brown on both sides, glue or tape a piece of blank white printer paper to one side of the cardboard.
Step 2: Cram It In!
Push the cardboard as far as possible to one side of the ziploc bag, then seal it closed.
Fold the excess bag towards the back (brown side) of the whiteboard, and secure it with paper clips.
And your done!