I had to Google Image TVR Sagaris, but I found some interesting images.
This looks like the "factory" style exhaust
Attachment:
TVR Sagaris 1.jpg
This looks like an aftermarket version.
Attachment:
TVR Sagaris 2.jpg
This is someone's project car.
Attachment:
TVR Sagaris Mods.jpg
I'm wondering why you think you need to make a rectangular exhaust. I assume it's for space requirements.
The project car illustrates something interesting. You can actually make a resonator using a end capped tube of the same size off of the exhaust. I'm not an exhaust expert, but I've always been interested in making mufflers/resonators for my own build, so I could control the space allocation for the system. I want my exhaust to exit at the rear and I don't want a big muffler (or even a hot exhaust pipe) running along the entry/exit point that the driver and passenger have to dodge.
If you look at the project Sagaris above, he has essentially build a resonator into the exhaust system. His system looks way overbuilt to me. You do not have to tie back to the exhaust flow to make one work. However, he did for some reason. It's not clear to me what his intent was.
Would a simple capped tube as a resonator work for your build?
Cheers,