In my classroom we have some with premade beanbags, just because we found them cheap on oriental trading. I used 2 pieces of corduroy, stitched it pillowcase style, and then sewed tunnels from top to bottom. Then I would put a beanbag in each tunnel and sew across it the other direction, all the way up. That way it was a bunch of squares. We had some kids that were good at picking holes, so I liked to have self contained beanbags and a way to repair. You can also use course sand I'd you use a tine stitch length and reinforce it. The quilted style helps distribute the weight throughout. I also recommend making some kind of washable slip cover in a favorite texture fabric.
If weight doesn't do it for your darter, pressure might. Look into neoprene pressure vests. You can buy them, but there are ways to make them too. Weight and pressure have different effects on proprioception. You can also try weights on different parts of the body, even weighted hats. Let me know if you needmore ideas... I'm a special Ed teacher so used to being budget concious and creative.