Here are a few options.
https://forum.ih8mud.com/threads/fj80-fzj80-lx450-lx470-front-seat-sagging-loose-netting-pocket-repair.1132859/
https://forum.ih8mud.com/threads/how-to-repair-sagging-seat-back-pocket-on-lc.813378/
https://forum.ih8mud.com/threads/my-sagging-seat-back-cargo-net-fix.450620/