I blow across the opening but find glass/ceramic flagons too heavy for my poor old bones. Gus Cannon seemed to use a galvanised oilcan that was light enough to go on his harmonica rack. In videos I've seen of Mungo Jerry they had one on its own stand like a cymbal stand at the right height for a seated player. Hammie Nixon played a one-gallon jug with Yank Rachell, I would guess anything bigger would be too unwieldy.

RtS