What size for the insert?
I have 2 pairs of flowmaster hushpower mufflers that drone nicely between 2.5-3k. I found a shop that can install a cross over pipe for about 200 and make it removable for clutch, etc. It is going to be tight but with the skid off there is room to match the stock size exhaust pipe. Flowmasters are 2 inch, smaller bodied mufflers. Took out second cats... These were supposed to help with drone but its the off setting exhaust pulses from cylinders that the boxer has that creates the drone with two seperate banks.
https://www.holley.com/products/exhaust/mufflers/laminar_flow_mufflers/super_hp-2_series/
For 10$, I might try finding a home on the passenger side pipe instead of engineering a J-Pipe and trying to isolate the hZ that causes the drone.
For us boxster owners, we only have about 8 feet of exhaust so if you have ever dumped an exhaust under a Jeep or Truck, you know how loud it can be ever with 6 cylinders!