With that in mind, I made two large SIPs out of rough cedar fencing boards.
My journeyman carpentry skills were put to good use; it wasn't a simple project.
The first step was to mill the boards so the long edges were square. Then I glued, and clamped, them up into large panels using biscuits.
The water reservoir is made by lining the box with 4 mil plastic. The soil screen is boards drilled with 1/4" holes that rest on a wood cleat 3" above the bottom of the reservoir. The wicking chambers are strips of plastic cut from a 5 gallon bucket and then turned into small rings.
Each planter measures 70"x16"x14" and holds 6 cubic feet of growing medium. It took roughly 20 hours and $80 in materials to make and install both of them.
I'm about halfway done with a major project to retrofit my existing 30 Rubbermaid tub SIPs. Instead of the plastic bag cover and angular trellis support, I'm cladding them in panels made from inexpensive brown UV resistant tarps and cedar. In addition to looking better, the new design is much more functional. I'll have more to say about the changes in another post.
[Updated 9.25.10 -- There has a been some cupping of the fence boards because of their high moisture content when I bought them. It's mostly aesthetic, nothing that makes them fall apart. In the future, to avoid this, I'm going to use the more expensive kiln dried cedar.]