Rear Speakers too boomy

[Scruffy]

I currently have my rear speakers in the bulkhead (kombi) low down behind the rear bench seat. They are cheap and nasty and about to be replaced with something more worthy of my new head unit. The rear wheel arches are boxed in and the rear speakers vent into this void, if you can imagine that. Thats quite a big space, and the bass can be a bit boomy. What do you suggest to fill that area, that will hopefully tighten the bass?

I've got a lot of 35mm Dacron insulation left over. Would that do the trick, or something denser?

 

[Fish]

Fast rings?
@Absolut5

 

[Scruffy]

Cheers @Fish, I’ve never seen those before. I thought about pods to cover the speakers but I’ve no way of accessing the area behind the speakers.