All I'm looking for is a tun, for batch sparging, that will drain the whole lot if I keep the valve open. Now, with my current 120qt rectangular cooler, there is a port on the side. I have a diptube that more or less goes to the bottom. So, it will siphon and drain properly, provided you don't interrupt the flow when the liquid level is below the valve. If you do, the siphon will break, and you can't get more flow, even when there is a couple of gallons left in the tun. I could install a bulkhead in the bottom of any container, but then there would be a flange that won't allow all the fluid to drain all the way. Slightly sloping bottom would minimize this effect.
I'm really interested in a container with the right geometry as a starting point. Insulation and all that would be done as diy.