Update March 23, 2010: The top photos construction will work, but this more recent version would be even more simple to make. It will work in any watertight container. Check out this post: The laptop of fresh food.
The do-it-yourself sub-irrigated planter (SIP) (erroneously called "self-watering" in the retail market) is a configuration (top photos) from the Rooftop Gardens Project in Montreal. I made it using only a box cutter except for using a drill motor to drill a drain hole in the side. You could easily make the hole with a hot poker if you don’t have a drill.
The material is expanded polystyrene panel foam insulation (often erroneously called Styrofoam) just like the tomato plant cell pack on the right. The panel foam came in a package of six (6) 3/4"x13 5/8"x48" "boards". That's enough to make six or more grow boxes of this size.
Note that I since realized that the Rooftop Gardens Project SIP grid is made from recycled plastic signs (ex. political campaign or real estate signs).
The support grid forms 9 spaces. You can see that the 4 in the corners are left open. These are filled with tamped down potting mix and create 4 "soil wicks". The other 5 spaces under the support platform form the water reservoir. You simply add water through the fill tube until a small amount flows from the overflow drain hole and then you know the reservoir is full.
The box is a 14 gallon, (23.9x15.9x12.2 in.) bronze Rubbermaid Roughneck tote box. The next time I would use the same type box only in the 18-gallon size (23.9x15.9x16 in). This would increase the water reservoir capacity. Keep in mind that this will also increase the overall weight when full.
The fill-tube is 1" inside diameter vinyl tubing.
This is as simple a design as I've seen for a DIY sub-irrigated grow box. The people at the Rooftop Gardens Project are experts in the science and art of rooftop gardening. They know how to make things from recycled materials in the simplest way possible. I consider them the world leaders in this field.
This grow box is made in the fashion of the commercially available EarthBox, Tomato Success Kit and the Garden Patch Grow Box. There are others along with several D.I.Y. versions on the web.
As Bruce in Chicago has pointed out, the Rubbermaid tote boxes are not made from UV resistant plastic. This will shorten the life of the boxes. You should question the supplier of ready made planters about the UV resistant qualities of their plastic and their estimated useful life before buying.