I'm guessing you want this:
https://www.summitracing.com/parts/dou-dec300aks
Can put it right before the muffler. It's a leakproof design unlike the butterfly valves, do some research on both. If you for some reason still want a butterfly, i understand raceready makes good ones. On that note, buy a quality cutout - don't spend all the time and effort installing a junk one from ebay that's just going to break or otherwise have reliability issues.
Going with an aftermarket muffler that works with the stock valve requires the canflap module to control the valve on the 235i, which adds $300 to the already high price. IMO a cutout before the stock muffler is the way to go, I'll be installing the Dougs unit next spring and will be sure to post pics and video.