i use a water butt and a guy i helped setup only has a side path to store water, so he has 3 X 210 L water butts linked with some 1 " wastepipe, you can buy tank connections for "hot water header tanks" that are ideal to fit on the water butts
he has them slightly staggered with the far end 1 filling up, when its half full it goes into the next and so on, t, it works ok and he seems happy