Here are some ideas for you!
Here's a post that might help - https://www.wikihow.com/Make-a-Pillow-out-of-Jeans
I have never done this, but seems to me, all you would need to do is to Hem/Sew the waistband and legs closed. Then open up the zip and stuff the whole thing with whatever stuffing you desire. It should then give you a pliable pillow...
If it were me, I would get quilt stuffing and fill a whole blue jean. Sew each leg shut at the bottom and fill each one as full as you want it, then fill the top and sew it shut also. You can buy fabric glue to close the openings, but sewing them would last longer. Hope this helps and I hope you have a pair of jeans the size you need.