You can sew cloth ropes on the four corners of the quilt core and on the four corners of the quilt cover. When you put them on, you will tie a rope so that you won't run away. I have seen this kind of quilt before. Of course, the size of your quilt core and quilt cover should match, otherwise it will be useless.
Check the quilt cover and quilt core. If there are belts at the four corners of the quilt cover and cloth rings at the four corners of the quilt core, it will be fine to tie them together. If the quilt cover itself doesn't,
First, nail a slightly larger button on each of the four corners of the quilt cover, and nail an elastic band on each of the four corners of the quilt cover, which is available in supermarkets. Note that both ends of the elastic band are sewn together, so that when the quilt is put in, as long as the buttons on the quilt are buckled with the belt on the quilt cover, the quilt will not run away.
It is also possible to sew adhesive tapes on the four corners of the quilt cover and the quilt core, instead of completely sewing on the quilt core, only one side of the adhesive tape is sewn on the corner. It's a little more convenient than tying a bandage.
If you don't want to sew, you can use a safety pin or be nailed, provided that your quilt is not that kind of duvet. After all, the duvet may run down if it has pinholes.
There is also a quilt fixing card that directly locks the quilt cover and quilt core together, but it is not suitable for thick quilts. If it's no trouble, you can sew the quilt cover and quilt core together directly like your parents, or you can just sew four corners to fix it. Cut it again and draw it again when disassembling and washing, and all the threads will come out.