Quilts are normally finished with binding, and not stuffed like a bag. The binding goes along all the edges, sewn to the quilt top and then the quilt bottom. For something being stuffed like a bag, usually the answer is to sew almost all the way around while it’s inside out, leaving a small gap that would let you put the stuffing in, turn it inside out through the gap, and then that little part is sewn closed (tuck the raw edges in).
Stuffing material - maybe set aside anything cloth that you would have tossed and cut it into strips? Towels, socks, tshirts, that kind of thing. If you know anyone who knits/crochets, maybe they have leftover yarn ends they could give you. Definitely do the actual quilting though, so whatever you stuff with doesn’t end up shifting.
This blog post includes a great tutorial on making binding from strips: https://www.jozmakesquilts.com/blog-and-tutorials/how-to-bind-a-quilt
The top looks great and is such a fantastic use for old denim!