Ok - lotus bog . . . . . they don't care what shape it is or how wide at the narrowest point and that sounds plenty big to me. Just dig down about 18" (or however deep you want), line it, fill it with dirt and plant those tubers. The water above the crown can be much deeper than 2" . . . . in the wild they grow as far as 3' underwater . . . . and I bet Joyce has some growing deeper than that in her pond at work. I always grew mine with at least 8" of water over the crown because then I could forget to water for quite a while before the dirt got dry. (Would have gone deeper but they only make pots soooo deep.)
I don't know what zone you are in, but my guess is that the lotus in pots even halfway in the ground will be just fine. As long as the tuber doesn't freeze solid, they should be ok.