I know there is a woman on Etsy who makes weighted therapy blankets. Hers are quilted but maybe she can make the weighted part and use and thin fabric and you could knit a cover for it out of whatever you liked. Here is a link to her store..
If that doesn't work, just search for "weighted therapy blanket" in Etsy and you will find her.
Good luck! I'd love to see it when it's done!!
Late at night, when the family's all asleep,
you will find me counting stitches
instead of counting sheep!
I'm on Etsy when I'm not knitting...
I'm Hartleystudio on Ravelry too!!