Minky adds some good weight, and wears very well, too.
Another idea is to make a "bullseye quilt," with layers of frayed fabrics on top of each other.
We make a "shingle" denim quilt on our longarm. It starts with the quilt backing and batting. Then we lay a piece of denim on top, and quilt it down. Then we lay another one on, so it overlaps the first one by about an inch. Just keep adding until you cover the whole quilt. I just weighed one, and it's 10 lbs, for a small twin size.
Maybe you could start with 2 layers of flannel for the backing. Then add two layers of batting.
If you get too thick, you'll need to raise the presser foot height on a longarm.