Just speaking from experience here...
I built this box and flush mounted the driver. Because of flush mounting the driver butted up against the 4" of rockwool.
I drilled 4" holes, can't remember how many but several, in the board and put 2" of rockwool on front of the board, and 2" behind the board so that the vent on the subwoofer could breath. I wanted to keep the 4" of rockwool behind the driver and that is how I figured to do it.
Performance wise I believe it had zero impact on anything audible. My sub still had steller performance. Here is an in room frequency response graph using REW. As you can see the subwoofer had very strong output down low.
and it sounded quite smooth through out it's frequency band, except some serious room nodes.
Like mike c said though, definitely more work.