How to build a muffler

[D8R]

Have looked at net about this topic but normaly only get info about how to take away low base tones. That´s oposite of what I want to my CAT D8.
Engine is a 4-stroke Honda GX100 and space available is
width 5 1/2"
lenght 4"
height 4"
exhaust dia is 22mm (scale)

Any suggestion before starting cutting steel? Weight is no issue here so it can be built with thicker walls if needed.

Dan