I see your point Terrier. I would worry about having large bowls of water sitting out, even with pebbles, as they attract mosquitoes. I guess it is fine if you are not able to water every couple of days.

I usually think of compost as recycled vegetative material. I have a compost bin with old leaf and plant material, some grass clippings, vegetative waste from the kitchen, coffee grounds, etc. I do use some peat moss, but not much. I even add cow and/or chicken manure to the heap. I've found that compost really does help retain moisture, especially if you add vermiculite. The square foot garden website gives a "recipe" for the mix which is the initial step in creating the soil for container. Afterwards, I just add a couple of shovels of compost when adding new plants. We had a hot spell last year and nothing dried out.

The car tires seem like a great idea. With the square foot method, the tires actually will waste space as everything is planted in a grid, and with a circular tire some of that space changes - but you can improvise of course.