348 / F355 Spider top boot foam disintegration solutions?

My top boot has the failing foam inside the liner that disintegrates and showers tiny black/brown, gritty dust all over everything. What solutions have been found to get rid of this?

I have vacuumed the top of the liner, but there's still dust coming out.

 

I carefully ripped open an inner seam, leaving the exterior thread unbroken. Once that seam was open, I cut the inner liner for a short way along the inner seam. I made an opening big enough to get my hand and forearm through. Then I carefully separated the foam from the flannel. The fabric is layered thusly: flannel, foam, and a white loose-weave fabric, kind of like cheesecloth but looser. The foam is stuck to the flannel, but will separate fairly easily. I separated it all with my hand holding the foam down while pulling up on the flannel with my other hand, then once it was all loose, I tore the flannel/cheesecloth layers away from the seams. I removed the foam through the hole I'd made, then mimicked the machine stitching by using the existing needle holes (a bit tricky, but doable). I pulled the needle and thread through the cover, captured the unbroken outer thread, then put the needle and thread back through the same hole. So the seam looks factory stock from the outside, and it's fairly unobtrusive on the inside. When I sewed it back together, I captured the cut edge of the flannel as well, so there's no holes in the cover.

I removed the foam from the two large sections using the above method, but I left the two smaller sections alone. They still left a bit of powder when I put it on, but not nearly as much as the first time.