Making a little more progress.
Got the outer housing initially shaped, with a bit more tweaking to do yet. It's 1/16" for strength, but that also makes it a bit tougher to bend and shape. I've got three chambers in it. The first chamber is primarily a surge volume/attenuation piece, while the other two are primarily attenuation pieces, similar to a pair of glasspacks in parallel. Below is a test fit.
I've drilled a couple hundred holes in the tubes, along with a few dozen slots to allow noise/vibrations to interact with the fiberglass and SS packing material around the tubes. Still have a similar number of holes and slots to cut yet before I can start to weld the end caps on. I also need to polish the exit pipe so it looks similar to the stainless tip on the other side of the car.
Right now the weight is 7 1/2 pounds. My target is 10 pounds (six less than the muffler it's replacing).
I was looking on ebay and found some inexpensive sound dB meters. I'm going to order one, and then hopefully be able to make a reasonable judgment on the comparative noise reduction capabilities of this and the other side muffler.