We had a weighted vest made for difficult child 3. He hardly ever wore it, but the design is simple. I suspect you could modify it for a blanket.
The vest - it was a standard vest design, a bit like a photographer's vest. It has pockets in it. Long, thin pockets as if you are putting a thick felt pen in each pocket. Actually, each pocket would take about two to three felt pens. But instead of felt pens, each pocket could also take a small sandbag - long and thin.
The vest was made of heavy cotton, not quite canvas. The sandbags were made from calico. It was all double-stitched (the vest was made with French seams, so the seams wouldn't rub) and the bags were very strong, absolutely no leakage possible. Each bag weighed 200 g.
The vest did up the front with velcro. There were two pockets on each side at the front, and four across the back. The pockets reached from the hem of the vest up to armpit height.
We were given choice of colour and chose royal blue, to match his school uniform colour (not an issue any more). We were also told to adjust the number of sandbags depending on his weight and what weight he seemed to prefer.
I guess it would be easy to adapt a blanket by putting on pockets so you could add or subtract sandbags in much the same way. It should be possible to make something like this, if cost is going to be an issue.
Some suggestions (from my mistakes) - use a strong fabric. Our first attempt, I used an old school shirt of difficult child 3's and the fabric just wasn't strong enough.
Sandbags seem to be the best option for weights - they are flexible (like those old snake-like draught excluders people make to put against a door). It's amazing how much weight you can pack in. It's also chemically inert - initially, I used lead plates which I folded and hammered myself, from scraps of lead I scavenged. Even copper - it would have corners which would dig in and be uncomfortable. And the metal close enough to be perspired on - would have discoloured the fabric and could even have been toxic.
With the sandbags, you can move them around and change them, depending on what effect the child wants. it gives them an even greater sense of control over the sense of pressure.
If you use weights, you could use a strong yet light fabric with weights on the corners and the edge, but still light enough to not be too hot in the middle of summer. For us, summer was a big problem, as it's hard to have the weight without also having something that gets hot. So a fabric that breathes is a good idea.
Apart from that, you could use any fabric (not stretchy) that the child likes the feel of. If you're going to make one (blanket or vest) I would get the child to help choose the fabric.
Marg