I made a custom 4-2-1 exhaust for my GS a few years ago. I used a mac 4-1 headpipe that had a smashed collector, collectors from an unknown rusty 4-2-1, some scrap pipe, and a supertrapp muffler.
I had several goals in mind at the time. They were to:
1. Create a clean looking system that didn't diminish power.
2. Have a distinctive, louder than stock, but not annoying sound
3. Keep ground clearance and cornering clearance
4. Maintain access to my oil drain
Since I know you all like pictures, here's what I came up with
First Pipe
One of the two 2-1 collectors
The 4-2-1 collector set up
1st pipe fitted
All the welding done
Still get to my oil drain!
Done (I have since touched up some of the welds a little better and repainted)