I did this a few years ago using a then "new" product from the Pak-It Rak folks, I think they called it "Dial-A-Tone" or something like that. I don't know if they still sell them or not.
They provide a template/adjustable baffle.You drill some holes in the end of your mufflers and attach the baffles. You can then rotate the baffle to adjust the size of the holes you just drilled, thus giving you anywhere from OEM quite to "Geeez, what kind of exhaust do you have on that thing!" settings.
I liked the sound produced with full open, (BTW I don't think anyone does this type of modification expecting it to "Sound Like A Harley", That is just stupid, It's a 6 cylinder for Christ's sake!). I did not like the drone produced at Highway speeds. I eventually switched to a complete Torqmaster Muffler and Loopz exhaust system, I'm very happy with that setup!
Stock muffler with exhaust tip removed.
Template/Baffle taped in place to drill pilot holes.
Retain screws in place. These can be loosened to rotate the baffle and adjust the "tone" of the muffler.
All seven pilot holes drilled, (use good drill bit, those mufflers are made of high quality steel, stainless I think!)
Drilling is finished on Muffler, larger holes are 3/8" if I recall correctly.
Installed baffle after removing "knock-outs" that where used as a template.
Then just reinstall your exhaust tips.I had Turndowns and was able to adjust the baffles without removing the exhaust tip.
Bike was not extremely noisy, but it did turn heads at 100% open, Highway drone noise was more tolerable at 50% open but I still didn't like it on longer trips, was fun as all heck around town though!
Like I said I eventually switched to Torqmaster mufflers, but the baffles were certainly a cheap way to find out if I even liked a more open sound and even how open I may like it. With the baffles fully closed the bike was as quiet as OEM.