EDIT: I was writing my long-winded reply when Roger posted his comment. We are on the same page.
You want the stuffing to fill the void as evenly as possible. With your smaller stuffing sausage, it appears as if you will have some unfilled spaces, mostly in the corners and in the lower section of the expansion chamber. Since we have the pillow stuffing already and you seem to know how to get to Joanne’s, maybe you should just make a pillow custom fit to the chamber?
My approach would be:
1. Create a paper pattern based on the section of the horn to be filled. It should look like a tall, truncated right triangle solid that would fill the horn perfectly. I would increase the dimensions by 1/4” (h x w) to assure it is gently compressed when installed.
2. I would use very light nylon mesh. It may be used for filter fabric and is usually slightly stiff and see-through. You should be able to breathe through it as easily as grill cloth. Cut the cloth to your pattern and sew into the pillow shape. It doesn’t have to be pretty but it should be strong enough.
3. The pillow doesn’t need to a top; the stuffing will stay put without one. Leave the top open like a paper grocery bag. I would sew a Velcro strip to the top so I could attach it to the enclosure behind the driver. I would also include a safety string in case the pillow slides to the bottom of the enclosure and you need to retrieve it.
The plans call for 20-40g of stuffing. I would listen to the speakers in their intended locations first with no stuffing. I would then load the full 40g of stuffing to get an idea of what maximum damping sounds like. I would remove the pillows/bags and remove the stuffing. Weigh out 20g and evenly fill the bags, paying careful attention to the corners. Install the stuffing bags and test. You can now remove the driver and add another 5g (25g total) from the top, reinstall the driver and test. Repeat 3 times until you have used the full 40g of stuffing. Decide which version sounds best to you and revert to that stuffing load.