That matting should work fine. Keep in mind that the main reason for fiberfill is to lower the box Qtc, making the box appear slightly larger to the driver than it actually is. If the matting is fairly dense, it may not act the same as an equivalent mass of pillow stuffing would. A single layer should be more than sufficient. The main thing for SQ with your enclosure is to brace the box heavily so that minimal flexing occurs. Stuffing will only slightly impact the overall sound, especially since you'll be driving the woofers with frequencies of 100Hz or less.
