Ok, so you need to trim one out and find your own fill... (home depot)
When I helped Steve with is pro 8" MTMs, he used AP-vents just as you describe there, about 4" diameter. Just shove a tight open-cell foam plug into the back of it, and voila!. I've done that more than once prior, and it works.
Comments
Some of the most interesting internet fights I have seen are when people disagree on "damping" vs "dampening".
What a world we live in
InDIYana Event Website
InDIYana Event Website