Advice finetuning generator exhaust noise

I own a Onan Microquiet 4000 which I am extremely happy with. I have it roof mounted in a custom soundproof enclosure and for a generator it's size, I'm pretty sure this is the quietest unit I've ever "not" heard. The only noise that can be heard from more than 20 feet is the popping noise and general exhaust coming from the stock muffler and tailpipe. Now, I understand this is totally acceptable by most standards, but being that I'm some sort of anal retentive perfectionist (and an engineer by trade), I would like to find a way to reduce this tailpipe noise so that the final product is the same coming out of the back end of most any automobile. So here's my question: Seeing as I have enough room on my roof-rack to mount anything I like, is there any reason that adding a new automobile muffler off the tail end of the generator's exhaust pipe wouldn't eliminate this noise? I had two thoughts. One, simply fabricate an adapter so that the auto muffler would bolt to the end of the tailpipe or, weld a 16" section of 1-3/8" perf pipe to the end of the Generator's 1 3/8 tailpipe to break up the popping percussion and then mount the auto muffler over the perf pipe so that the perf pipe fits inside the muffler, sealed behind it. If anybody has had any experience with this, I'd sure love to hear your thoughts and/or ideas! Thanks.