Here is my jumbo version of a soda bottle self watering container.
Start out with one 5 gallon water cooler bottle, preferably one without a built-in handle.
Drill holes for soil aeration along the sloping side of the bottle and along the spout.
Cut the bottle into 2 pieces, the soil container and the water reservoir. Drill an overflow hole in the water reservoir for drainage.
Where you divide the bottle will depend on the distance between the top of the spout and the end of the sloping side on the bottle, where it meets the straight side. This distance will be the height of the water reservoir. I, of course, did not measure this distance with a measuring tape (I eyeballed it) so I got it wrong. To compensate I cut off a portion of the spout. Ideally you want the spout to rest flush with the bottom of the water reservoir to maximize stability.
I ended up cutting off a little too much from the spout so to bridge the distance between the spout and the bottom of the water reservoir I added a terracotta plant tray. Avoid my mistake! Measure twice, cut once.
I placed the soil container over the water reservoir, added a small tomato cage for plant support and finally added the soil. The tomato cage went into the planter before the soil. I had the spokes of the cage go through some of the aeration holes so that the cage rested at the bottom of the water reservoir. Having the cage sit so low in planter makes it much sturdier than if it was just stuck in the soil. Voila! The latest edition to my rooftop container vegetable garden.
Update: Algae will grow in the soil and water reservoir if you don't block the light so cover it with paint or duct tape before you put it outside.
The only monetary costs involved in making this container were the tomato cage and the soil. Everything else I had on hand.
1 Tomato Cage: $1.63
1 Bag Self-Watering Container Mix: $9.95
Running Total: $81.56