The full length, bass to tip of the headstock is 134cm
Been looking, found a Warwick Buzzard case that was long enough, but an odd shape which might not fit (in fact, I was puzzled how it fits a buzzard), and a few 88 key cases. There is an Flying V bag that is long enough but it tapers to the neck (as you would expect from a V), which is where the sei is the widest.