To finally answer your questions, Mike:
The bracket for the muffler is made out of a 110mm long piece of 20x20 stainless steel square tubing. This has been cut in a taper to make the muffler point slightly up at the rear end, and welded directly to the muffler.

Thanks for the pics and sketches Finn.  Excellent job on the mounting bracket.  I have a slightly used hi-flow that I want to install on my next project.  I'm gonna try to pop rivet a bracket onto that muffler.  It has a slightly larger baffle with more perforations.  I ran it for a short time, but it doesn't have provision for an O2 sensor, so I took it off to do some testing and just never reinstalled it.  I need to modify that one with your bracket and an O2 sensor.

Are you planning to do any sort of flow testing on your muffler?

Finn, I simply set the muffler in my test bench and measure how much depression I can pull with two shop vacs.  I don't use an orifice or the inclined manometer, just pull vacuum in the normal direction of flow and see how much depression results.  The muffler with the lowest depression should have the best flow.  Since we have two completely different vacuum sources, I don't think we could learn much from comparing data.  But you could certainly use your bench to compare your before and after results.  This old post should provide the necessary info.

http://suzukisavage.com/cgi-bin/YaBB.pl?num=1605160449

I use the O2 sensor to help me dial in my carburetors.  I am always trying new things, so instruments come in handy to ensure I have the best tuning for a particular setup.  I have found that air/fuel ratios around  11.8:1 to 12.5:1 usually result in the best WOT acceleration.  I use the acceleration data along with the A/F ratio to arrive at the best size main jet.  The A/F meter also provides interesting insight into carburetor behavior, how the A/F ratio corresponds to throttle position and load.  It doesn't behave like I thought it would, so it's an excellent learning tool.

The instrument I use is an AEM 30-4110 Wide Band UEGO with Bosch LSU49 02 Sensor.  I made my own instrument housing.

The center tube, or baffle is shown here, along with the end plug ring.
During the drilling of the many holes, I forgot how many was neded, and I ended up with 120 holes on each side of the baffle plate. Since the hole size is 4.5mm, and the target is 108 holes of 4.34mm, I selected to block off 20 holes, to end at 100 holes each side.
Nothing is pretty here, I just wanted to get the job finished in a couple of hours.

Here is the finished header/muffler project, this is good, because it means that I no longer need the old engine as a welding fixture, but can start to dismantle the head, cylinder and piston, and rebuild it as the high comp, funky DRcam, ported head and mikuni carb fed, beefed up clutch, high flow oilpump, modded cam chain tightener and high flow exhaust engine.

Did I forget something? Gotta be a blast!

That is an exhaust that I have been dreaming to build for the last 10 years!  It looks great!

I have not been able to locate anyone that will mandrel bend a stainless pipe for me locally.

I bought a big rosebud torch that uses propane for fuel - I am going to try bending a pipe with sand inside.

That is an impressive increase in flow.  Thanks for taking the time to run it up on the bench.  Twice the flow with only a slight increase in noise, what's not to like about that?  Next time I set up my flow bench I'll have to see if I can do some tests with the orifice and inclined manometer.  

Cool the way you are covering holes with the sleeves.  You might even be able to make the sleeves adjustable so you could fine tune them.