If you have experience with this and know how much water any volume of soil mix can hold, then this is pretty viable.
Don't fill the bottom with pebbles, gravel or any of the sort. It will create a perched water table at the interface, giving you even less depth for roots to grow without suffocating in water. Fill it up entirely with the medium that you intend to grow in. The coarser, the better.
Don't leave it out in the rain though.
