For this I am using the 330 gallon metal framed poly tote tank. These are used to carry just about any kind of liquid imaginable. From paint to vegetable oil. If you look around industrial areas you can usually see them stacked out back. Most places will give them to you just to get them out of their way. Try to get as clean of one as you can find. Once you have cut the top out you can hose it out and treat it with a 15% bleach solution if needed.
Start be removing the two top cross bars. Then cut the top out. I use a compass made from a stick to draw a circle on the top to cut from.