I actually used a round pan! Using a rectangular pan is a lot easier and will turn out a lot nicer, but with a round pan you just have to be careful to keep two of the edges straight for the sides, so that when you roll it up it’ll still look nice. It’s pretty simple to do: once you pour the beaten eggs into the pan, just use your spatula to scoot the egg back a bit until it is straighter.
Worse comes to worse, you could always just make it round, and then fold in two of the sides like a burrito. The edges will just look a little odd, but it won’t be too bad.