Commercial polycarbonate boxes
for growing square watermelons run about $85 plus shipping. But you can make one for about $8 out of plexiglass. You'll need six rectangles of 1/4" or thicker plexiglass or other translucent plastic. Free and super cheap plastic pieces can be had from sign-making shops--ask for scraps! Or try to find a plastic supply warehouse that caters to farmers, these will usually be the cheapest source for plexiglass. If you buy plexiglass scraps, ask to have them cut into rectangles 8.5" x 8". If you are doing the cutting yourself, you can use a table saw, band saw, or a hand saw.