I’m dealing with drone issues too. I have an eBay can that is similar to the Megan unit, except for even less volume and quality! I think there are two parts to this problem.
1. Reduced exhaust pipe length
2. Reduced volume.

The OEM muffler has a big chamber and loops the flow.

SRT6 guys complain less about drone. Could be because of the added length of the pipe going to/from the res, compared to the NA exhaust. I think you could solve some of your problem by taking the exhaust on the longest route you could fit under the car with a 180 degree bend that follows where the SRT6 resonator goes. Or use a 180 degree muffler from Flowmaster or spin tech, which would also help your volume.

I’ve added a branch pipe and a small res. Didn’t do much. My exhaust guy didn’t really do what I asked with the branch pipe, so it might work if your exhaust guy listens. Bolt in silencers for the tips did a lot.

I think two 4x9 offset oval cans, mounted vertically to a Flowmaster super 10 (acting as a y pipe) would give you the added volume you need. This is the setup I’m thinking about. ends up being a 9” wide and 9” tall package that can tuck up into the cavity like the OEM unit. (In theory)

Also considering putting the stock muffler back in with some different tips. I know it works. Doesn’t drone a bit. Too quiet at WOT, but nice when getting around town.