I have a system that uses a series of 6 55 gallon plastic drums (they were
lactic acid barrels from a local cheese company). I rigged them up using PVC
fittings and pipe dope/cement to build them in series of 2 tanks connected at
the bottom, then a spigot on each bank of 2. They are all connected at the
top, so when the rain (from my downspout) overfills the first 2, it dumps over
into the next 2, and then into the last 2. This way, if something fails, you
only loose 2 tanks, and are still operational.

Note-- that is one hell of a lot of weight, and since it's under gravity feed,
I had a freind build a steel rack behind my garage to keep them about 3' off
the ground.

Also, The "bung" on the tanks is mounted on the bottom, so you can get rid of
leaves and gunk if it collects there. It works like a dream. I usually have
at least one of the banks of 2 with an old pillow case of manure for occasional
feeding of the plants (manure tea).

Just a thought.

John

ps, yeah, my best friend is an engineer, and yes, my fiance and neighbors think
I lost my mind!