Here's the plan:
See? Simple to build, you may have a lot of the stuff on hand as scraps from other projects, and they are lightweight. This last bit is important if you want to move the container around, or place it on a deck or balcony. I have the one above (which is growing Golden Delicious squash) in an aluminum frame previously used to move recycling bins to the curb on trash day (scored free at a garage sale). I think it's going to be great at conserving water, and I think the squash is going to be lovely and grow enormously.
NOTE: One thing I forgot on the original plan was to indicate that you do need to make a series of perforations around the container, above the water tank section. Because this is a closed system, if you have a lot of rain, it will become a very muddy pool of drowned horribleness (ask me how I discovered this). So, make a bunch of holes to allow it to drain in case of storms that dump a monsoon quantity of rain in one go.