Art Ludwig says in his book that wrapping a tank with chicken wire and smearing concrete over it works, FWIW. The concrete clings to the chicken wire -- most concrete doesn't stick to plastic. I think it's like wearing an overcoat.

Ben, it would be aboveground (soil is far too rocky here for underground). The tanks I've looked at online are 5,000 gallons (just because 6,000 is the max here without having to get a permit). The tall ones are 8.5' in diameter, and 12.5' tall. The short ones are 11.75' in diameter and 7.25' tall.

Blast, plastics don't usually stick to concrete, or not securely, anyway. I use plastic for concrete molds, and don't even need a release agent.

Sue