Any tubing with an ID between 1/4" and 5/16" will work. The advantage of the 5/16" ID tubing is that you can always pinch it down if the exhaust outlet area is too big.

For example, here's a photo of an exhaust stinger that was pinched down. Notice how small the outlet area really is, but that's where the TT 42 GP in my Fazer ran the best overall.