I made yet another one. With another scavenged soda bottle. This one big enough to put a houseplant in. Trying to save my Cyclamen from the mold in its crown so I figured this is a good way to keep the top dry and the bottom well-watered. Turns out it was a good choice, when I got the plant out of its pot discovered it was very root-bound:
Next morn it had used two-thirds of the water I gave it and looks quite happy.