I played around with building some a few years back for a few of our rock crawlers. I was really surprised at how simply most of the high dollar mufflers are built. I have a good friend here in town that does a lot of muffler work and has a lot of flow testing equipment for them. After doing some research I found that they are very easy to build and in some cases I could achieve a good sounding muffler with the same or better flow characteristics than the brand named mufflers In some really small boxes. A few of my creations.