Here's a short video I did demonstrating the ability of acoustic cotton to absorb midrange inside of a speaker cabinet. These are the new "main" speakers in my system, which I'll write a post about soon.
The cotton products I used are 2" thick 2x4 panels from ATS Acoustics as found here on Amazon. As well as some Frost King cotton insulation which works really well for going around tight corners of braces and when near port openings as not to interfere with airflow. Its advertised as 1" thick, but its less than that. It is very affordable too, as found here on Amazon. As usual, I just glue it in using my favorite adhesive.
Stay away from fiberglass and rock wool products for ported speakers, as they would eject little bits of material into the air you breathe and aren't very friendly to work with.
Please watch my video, It is only a minute long and demonstrates how well cotton works. I'm not very good at making videos yet, but its a start!