Yup, like Cedric said, the wider, the better, and not very deep, 12" is good. Hardly any water over the top of the soil.
Although, when I repotted my full 1/2 barrel liners (20+ gallons size), the roots went all the way to the bottom, at least 18" deep.
And the tubers at the bottom were the biggest, the size of bananas!
We have absolutely NO shortage of half wine barrels here. (this is wine country out here on the North Fork)
I don't believe the story that they are shipping them to Europe.
Europe has just as many distilleries and wineries as we do, if not more.
So there would be no shortage of them in Europe either.
The only lotus that did OK in a somewhat smaller container was Baby Doll, a dwarf 'bowl' lotus.
That container was about 22" wide and 8" deep.
I don't believe in growing in clay.
I believe compost is richer, holds more nutrients, more like what they grow in their natural habitat:
the decaying composted organic matter that settles out into a thick layer of composted mulm on the bottom of ponds and bogs.
The lotus in the lotus pond at work literally clean the bottom of the pond by absorbing all that composted mulm. (which includes rotted plants, insects, koi poop, etc)
So by observation of the big lotus pond at work, I try to mimmick that in my container ponds here at home. And, for me, it works wonders, just giving them what they would naturally get.
Once again, my lotus have multiple daily blooms all summer. They start out having one or two a day, then when they really get going...at least 3 a day.
Somewhere here I have posted my lotus pots with 5-6, maybe even more blooms per barrel.
That was Momo Botan, which I killed by dividing and sharing, and not leaving enough for myself. :'(