I made a weighted blanket several years ago. Like another has suggested, I'd use your panel as a cover and, if you want it fastened to the weighted part, finish it by tying. You won't need to launder the entire piece that way. I can't imagine trying to handle the weighted blanket when it needs laundering and this would give extra life to the weighted part. If you want quilting as a feature, perhaps create that and add a single layer to the back so the weighted part can be slid inside.