Not all jellies have the same requirements, there are some which don't need round tanks and some which probably should. There are some that do well in captivity and some that most certainly don't. There are a number of relative constants though, including:
They need low flow and intakes MUST be protected.
Round tanks are recommended because they can get caught and injured on any corner, there are ways of doing this without a round tank though.
They need high lighting, they are photosynthetic after all.
They need live food, I think I usually hear about using enriched brine.
Any corals or tankmates have to be docile and soft/without hard edges.
Other than that, you really just have to find a jelly that's well suited to aquarium life. Something that will live in tropical waters, something that isn't an extremely active swimmer, etc. I don't know much about them but i've seen a few successful tanks which aren't the thousand dollar in-wall monsters that are used in commercial aquariums and such.
Look at some people's tanks who have kept them, I know there's a couple on nano-reef.com which I've seen, a cube aquarium was one and I think the spherical biorb was another, but it can be done somewhat less extravagantly than the extremely expensive option if you've got it planned out well.