In lieu of old tires or treated wood, I'm trying out old rotten firewood and twigs as building material for our potato box. The potatoes seem to be okay with their rustic accommodations. Every week as they grow, I add another level and a layer of compost. Hopefully it will reach three feet before harvest. We'll see if any potatoes roll out when we knock it over in a couple of months. I only used one potato cut in quarters, so it wouldn't take much to break even.