You don't need one if you are using the stock engine setup. I assume you want to eliminate the cutout?

I considered a flexible muffler that they put out to do just that but It was too expensive to justify buying it IMHO.

Here's what I did for my Saito 100. It actually helps the cooling having the side hole in place. Not a lot of exit area on this cowl and I did overheat till I cut an extra hole in the front (chin and "non-scale" location as described in the manual).