round trough 16 x 16 x 8"H
I've made 5 or 6 of these little (for me anyway) troughs. I bought a 12" diameter sonotube from Home Depot and cut it into 6" lengths on my table saw. To cast a trough, I filled each 6" high piece of tube level with sand, then pattycaked the hypertufa mix around that core. The troughs are cast upside down, with a 2" piece of cardboard tube as a drainage hole. Since the sides of the tube are plumb, the wet hypertufa mix tends to slump away from the core as I near the top. To stop the slump, I pack the hypertufa really hard with my hands and then wrap 6" shrinkwrap as tightly as I can around the whole form. It's not much, but it's just enough to hold the concrete until it starts to cure.
Like most of my troughs like this, I let these sit for 48 hours before working them with a wire brush and scraper. After that, they are sealed in plastic for 30 days to cure.
One day when I was demolding one of these troughs, the wet sonotube came out in one piece (a rare event). It was kind of an interesting snowman shape so I let it dry instead of pitching it. I used it as a core again the next week, resulting in the trough in the front of the picture below. It's a weird shape, but maybe it just needs the right plants. In fact, that's probably true of all of my troughs.