This instructable explains my efforts to make cheap growing boxes for square watermelons. Learn from my mistakes, grow cuboid fruits, and save money at the same time! You can also read about it at my blog: GoodCleanCrazy
Drill with bits
Cheap baling or fence wire.
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Step 1: Obtain Plexiglass
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.
Step 2: Cut the Keyhole, Drill Airholes
You will need to cut a large keyhole on one rectangle to allow the watermelon stem to poke through the box. Note the picture below. I use a hacksaw to cut the round part of my keyhole, but a holecutter bit would probably be easier.
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).
Step 3: Assemble the Box
Assemble your box and reinforce liberally with clear packing tape. The keyhole lid should be hinged on one side to allow the watermelon to be slipped in and out. I also twisted baling wire around it for more strength.
Step 4: Rotate the Melon As It Grows.
You grow the watermelon just like any regular watermelon, except that you also rotate the box every few days to ensure it becomes green all over. Otherwise you'll get a yellow spot on the bottom, and uneven growth.
Step 5: Personalizing Your Melon
If you wanted to "brand" your watermelons you can tape something opaque like aluminum foil to the inside of the box. This will create a yellow spot on the melon rind in the shape of your initial or whatnot.
Another way to personalize the melon is to cut holes in the box to allow the watermelon to protrude. For example, you could cut a smiley face or anything that doesn't weaken the box too much. The watermelon won't grow into a design with details finer than about 1/2 inch. Also, If you use this personalization method, you'll have to dismantle the box to remove the watermelon.
Please visit my blog for more projects and ideas: GoodCleanCrazy.wordpress.com