This is really, really easy to do. (Note: you may be able to use this technique to grow other stuff in the form of a square ( actually in the form of a cube) and apply the general idea to other shapes.)

Step 1: Materials and Tools

You need 6, 8" square, 3/8" thick (or thicker) sheets of polycarbonate plastic. (Lexan)

You will need 4 gate hinges and 2 hinged clasps with flat head machine screws and hex nuts plus at least 8 or more 1" to 1-1/2" long, thin wood screws and possibly a 36" length of angle iron or aluminum angle (cut to 8" lengths to yield four pieces) if you have thinner polycarbonate, a power drill and hole saw, and a hand saw or power saw such as a jig saw.

Step 2: Cut the Stem Pass Through

In the first sheet drill a 1" diameter hole in the center with the drill and hole saw. Then, using a hand or power saw, cut 2 parallel lines from the hole perpendicular to one edge of the sheet and remove the material between.

Step 3: Add First Set of Hinges

Use a box to rest the left side sheet, place the top sheet left edge over the left side sheet edge and fasten the sheets together as shown in the diagram with 2 hinges.

Step 4: Trim the Inset Sides

Measure the width of the sheets and trim each side of the the inset sides by this width.

Step 5: Fasten Sides

The edges of the inset sides will have to be trimmed the thickness of the poly you use so they can be inset and still keep the box square. Trimming these edges is probably the hardest step.

Drill pilot holes through edge of the left and right sheets into side sheets between them to accept the wood screws. Use 2 or more screws per joined edge. A piece of angle iron or aluminum can be used to beef up the corners if the polycarbonate you have is too thin. (If the poly is too thin, however, it may bulge.)

Step 6: Do the Bottom

Place the box on its top, add the last sheet for the bottom and secure with hinges as you did the top.

Step 7: Add the Clasps

Clasps go opposite the hinges on the top and bottom sheets.

Step 8: Usage

Plant watermelons. As soon they begin to grow large enough to place them in the box with the stem through the slit and hole, do so. Remove when ripe.

Step 9: Addendum: How to Grow a Watermelon

How to plant your watermelon

Watermelons come in about 1,200 varieties worldwide, divided according to season and seed production. China is by far the worlds largest grower of watermelon. Growing time is from 70 to 85 days for all varieties and growing season begins when all danger of frost has past. Watermelons can be grown indoors anytime of year but require extra amounts of space and constant temperature between 80 and 85 deg. F.

Seeds for seedlings to be transplanted outdoors may be started indoors 3 weeks before transplanting outdoors. Seeds are planted one inch deep in groups of 3 and seedlings thinned to the best 1 or 2 for planting. You can use this method with peat or potting soil to start seeds indoors. Seedless varieties must be planted alongside seed producing varieties in order to pollinate and set the fruit.

Hills for single transplants are spaced 2 to 3 feet apart while double transplants are spaced 4 to 5 feet apart. Rows are spaced 7 to 10 feet apart.

How to tell when your watermelon is ripe

Use black plastic to cover the hills and rows and plant through the plastic. Drip irrigation is sometimes used sparingly in cooler climates.

Use a combination of the following indicators to determine when your watermelons are ripe:

1. light green, curly tendrils on the stem near the point of attachment of the melon usually turn brown and dry;
2. the surface color of the fruit turns dull;
3. the skin becomes resistant to penetration by the thumbnail and is rough to the touch; and
4. the bottom of the melon (where it lies on the soil) turns from light green to a yellowish color.

Many watermelons do not emit the proverbial "dull thud"when ripe. For these, the dull thud may indicate an over-ripe, mushy melon. The above indicators for choosing a ripe watermelon are therefore much more reliable than "thumping" the melon with a knuckle.

Source: University of Illinois Extension Service