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.