I disagree that batt FG is a health risk. FG and Rockwool are similar material, similar health risk. Both have glue mixed in to hold it together. It only makes dust when being torn apart. Gentle breeze of reflex port isn't strong enough.
Alternatives for internal acoustic damping include Bonded Logic
shredded denim insulation, I think it's more powerful absorber than FG.
Pillow fill polyester wool can be bought at any sewing store. My speaker kit (not GR) had this specified in the build instructions. Acousta-Stuff
is a specially processed polyester fill that has more damping than plain poly fill. The two batches I bought over the years looked different from each other.
Sheeps wool has been used and the advocates are very enthusiastic. It is less powerful than FG but it has that certified organic, "natural," magical, Welsh lore aspect that makes the golden eared DIYers cream.
GR Research sells No-Rez
sheets which are very well received here on AC.
Since these all have different absorption coefficients, the amount and placement inside the box of each different material needs to be
experimented and tuned to serve the woofer's damping requirements. So when Danny says use 1/2 pound of FG batt, it's because that's the amount of FG that will do the job. If you decide to use wool or denim, then you would have to experiment with different amounts to learn how much is correct.
The amount and type of stuffing will affect the woofer response, so you have to decide on the stuffing before you advance to crossover design. And if you want to change a stuffing material then you should reevaluate the xo design. At least measure to make sure it makes the same response. Increasing the damping can reduce the SPL and overdamping reduces transient response.
After many DIY builds trying some of the above materials I use FG batt or 1" rigid FG panel now.