Sue how hard would it be just to build it out of concrete? My friends (both in Tenn and Portugal) have concrete above ground cistern that is all concrete made simple and no plastics and have been used for water for over 30 years (Tenn one). If I ever decided to build one I would probably go that route because:
1. I hate plastics period (chemical leaching,out gassing (learned that about food grade plastics from Nasa) etc..)
2: Simple to build
3. Highly reliable (from what I've seen)
4. Extremely Durable
5. Most (over 99%) of the places I visited in Portugal (on the island) are depending on concrete cistern as their only primary of water with all new houses are built with them and they have lasted for more years than I know of.

I always like to go with a reliable system that I can build it and spend as little maintenance and have the best reliability and durability and I don't care for plastics (did I mention I hate plastics,hate the taste as well)

Here's one outside of the place we stayed at (wife's parents) and she was born in this house and it's been there for a long time (she would kill me if I say how old it is because it may reveal her age )(hint it's wy older than the Tenn one) but needless to say it would last a lifetime with little maintenance and all concrete.