If the grip mounts to a picatinny rail, I would just mount a short section of rail under the handguard with recessed screws and nuts. If that is not possible, post a picture of the handguard and I can advise further
https://centerfiresystems.com/product/factory-saiga-12ga-2rd-mag-iz-222#product_detail
Used to be rarer than hen's teeth.
you can also cut down some other brands ( AGP, SGM)