A couple of the older posts alluded to these but you seem to have missed what they were talking about - Terms for the same thing: Helmholtz resonator, 1/4 Wave resonator, Branch Resonator Google Branch Resonator and the flick to the images results to get the idea This page has some technical ways to work out lengths http://mkiv.supras.org.nz/articles/exhaust_drone.htm
You could possibly fit a pair above the diff?
With some creativity you could make them adjustable fairly easily
I saw them, and it's a cool idea I would like to implement, but that would require welding and that's a skill I am not comfortable with just yet.
