Drill with bits
Cheap baling or fence wire.
Step 1: Obtain Plexiglass
Step 2: Cut the Keyhole, Drill Airholes
Then drill 3 small (1/16" or so) airholes in each corner of each piece. That's 72 holes total. It's a lot of holes, but it is VERY important (prevents the watermelon from cooking in the sun, then rotting in the box).