A kettle would certainly work, but if you want to get more creative and unique, think about what we had before plastics...ceramic! With some looking, you could probably find a large (10 gallon) pickling crock with a spigot that would work for you. You would have to seriously pre-heat it prior to mashing, but once up to temp it should insulate very well.
As far as batch sparging, the shape of the mash tun has little to no effect. It is fly sparging where the shape can have some impact.