You don't NEED a curved tank. I've kept plenty of jellies in a typical 'box'.
Also not all are cold water, some (like moon jellies and upside down jellies) do just fine into the mid 70's or even low 80's for temp.
Just make sure you have good flow and NO way air can get into the tank itself. Also, make sure the suction inlet or overflows won't suck them up. Lots of small holes over a very large area reduces suction at each point so they don't get stuck and die.