I gotta kind of agree with FishingRod, shouldn't take a boat load of engineering, doesn't even have to be drum tight for holding water, a decent leak isn't going to hurt anything, just wanting to hold water for a couple months.
We didn't do much engineering, just pushed up a berm on three sides. don't think we had 15k in the whole thing. cant remember tho.

All the really good ideas I've ever had came to me while I was milking a cow.