I am in the midst of a DIY JBL M2 build.
The factory originals are lined on 5 sides with a 1 inch (25mm) blanket (probably fiberglass). The frequency absorption coefficients are unknown and the density (pcf) is also unknown.
So far the recommended substitute has been a 1 inch layer of Wrap-on 16550 (plenum liner used in HVAC applications). This is no longer manufactured and there is no info on what the density or the absorption coefficients may have been. There are other plenum liners on the market but their specs vary all over the place so it is hard to know what a good substitute would be.
The other common recommendation is the "ultra denim" insulation. It's absorption coefficients are great (specified for a 3 inch thickness, so you need to reduce them by about a third). The density appears to be about 3 pcf. As luck would have it, this product is no longer manufactured and Lowes and Home Depot do not have any old stock (at least in my area).
So my thought is to get some Rouxal insulation (sometimes called mineral or Rock wool). I could remove the paper backing and tease apart about a 1-1.5 inch layer. I would apply this to the walls and keep it in place with some loosely stapled lightweight batting used for quilting etc. This would keep the rock wool in place and stop any fibers shedding. The specs on the Rock wool have good absorption coefficients and the density is about 3 pcf. (similar to the "ultra Denim").
My concern (and I am not trying to go off-topic), is what is the liner's "purpose"? Some 1) claim it is mostly to make the volume of the cabinet "appear larger" and you should be mostly concerned about the density not being too great, while 2) others claim it is to absorb the back wave of the woofer (and primarily worry about the absorption coefficients).
Again, remember this is a ported cabinet with sufficient mass and bracing. It is not a sealed cabinet and it is not a transmission line.
Thanks in advance