Take some pipe. Take some other pipe and cut holes in it. Weld it inside the bigger pipe. Weld a big aluminum box around the two pipes and cut off-set entrance and exit holes and attach to exhaust system. Who needs years of careful design study to create a muffler? Just throw some **** together and if it sounds terrible, who cares?

Seriously, go buy a Flowmaster 60 Series. They put time and effort into R&D for this stuff for a reason.