I admittedly don’t care if I change how the inside of my clothes looks so grain of salt here: go to a thrift store and get a sturdy fabric that’ll fit enough. Then cut a bit more than you need, maybe a quarter to half an inch extra at the widest part. Go bigger if you want the extra pocket space but it will look baggy then. Stitch it up around the edges, it’s on the inside so I personally wouldn’t care about what kind of stitch/thread/whatever as long as it’s functional.
Just make sure to double stitch so you aren’t putting a ton of stress on a small area of the old fabric or it’ll tear again.