http://986forum.com/forums/diy-project-guides/70112-internal-exhaust-mod.html
This is what you want to do. Pretty easy and if you can weld it's free. If not i'm sure a shop will do it for less than an aftermarket muffler. I've done the CRIOS mod and tried a Circut Works muffler. The link is by far the best sound and no drone. It's a great winter project if you live in colder weather.