Why not just add small resonators ??
I had considered that and, while searching for same, came across lots of discussion and youtube videos of people curing aftermarket-exhaust droning using these devices. One uses a phone app to measure the frequency of the drone @ rpm, then plugs it, along with engine size, cylinders, etc., into a formula which tells one how long to make the j-pipe.
The smallest resonators I found are 12" x 3" which will fit my 1-3/4" pipes and would probably be OK ground-clearance-wise if placed directly under the axle. Whether they worked would be guesswork, whereas the helmholtz resonator would eliminate or greatly alleviate the drone if I do the math correctly.
The length is critical in order to create a sound wave that's 180 degrees out of sync with the main drone, therefore cancelling it.