I had two resonators installed in mine, the drone was still there, all they did was make the exhaust sound tinny and weak. The wagon is a speaker box, and you are inside it.
The only exhausts I have heard that don't resonate/drone are the Fujitsubo Legalis and Nismo for the M35, surely there is something similar around cheap you could pick up S/H?
With enough trial and error you could weld in a Helmholtz chamber, as Fujitsubo use. It is a tuned dead end muffler, which can take out certain frequencies of noise, specifically that freeway drone.