I have made a couple mufflers using products from Cone engineering.
https://www.coneeng.com/index.html
I have a similar header set up on my 964 and built my own muffler. I used two of their 4" x 12" muffler bodies welded together and the end caps. Connection is via V-band clamps. I used their merge collector on the headers as well.
.
The inside of the muffler is a x design. All the straight sections internal to the mufflers were perforated tube. I used high temp packing made for off road motorcycle mufflers to stuff the case. drawing of internals.
The first one like this I made was for my 996 turbo. I used the 10" wide cases for it.
So you have a range of widths you can make the muffler and the tips.
I think they sound good and are not stupidly loud. They cost hundreds to make instead of thousands to buy.
john