Ditto on the Magnaflow solution. I used a Che muffler on my Spyder and while it sounded great, the fitment was not good. It rattled against the frame rail, so I bent it a bit, then it rattled against the cat, so I hammered the cat heat shield in a bit, then it rattled against the bumper cover, and then my engine blew and I sold the car before I figured out how to get rid of that rattle.
I got a 4x9 Magnaflow 11236 for my FR-S for 60 dollars, and had it welded in for only 70-80 dollars IIRC. It's very light, and almost stock quiet on the FR-S, which sounds pretty crappy no matter what you put on it, but it lets a nice clean growl through. The Magnaflows seem to have very little restriction while reducing the sound a lot, if you read some comparisons, and doing it this way is cheaper than buying a whole catback for the same amount of money then spending a bunch of time trying to put it on.