I've never tried this idea, but think that it should work. What if you got two five gallon buckets, and placed one inside the other, with some kind of spacer to keep a decent reservoir in the bottom bucket. Drill a watering hole and overflow hole in the side of the bottom bucket, just below the level of the upper bucket. Drill a hole in the middle of the upper bucket and constuct some kind of wick to insert. Perhaps buy some fairly thick cotton rope. Fill the top bucket with planting medium, but lift the wick so that it rises near the center of the soil.
Wouldn't that work as a self watering arrangement. Seems to me that as the top dries, that water would wick up the rope path and into the soil. I've also thought of trying that same approach, but with bottom trays. Set the upper bucket in a bottom tray, but spaced up on pebbles, brick pieces, or whatever. Then drill a hole in the bottom of the planter and insert the wick from inside the planting pot, down into the bottom tray filled with water. Sees that mosquitoes would be a problem with that arrangeent though, with the water so accessable. Perhaps would be better to let the planter overlap the bottom tray instead. So as to seal the water off from critters.
As a last variation, it would seem to me that most any kind of container with a wick inserted could work as a self watering arrangement. Take a regular 2 liter coke bottle for examle. Drill an appropriate sized hole and insert wick material. When filling the planter with soil and plant, simply bury the coke bottle with wick while leaving the opening just above the soil line. Just water through the coke bottle opening, and allow it to slowly wick to the plant.