I take it you are wet molding the pouch? If you use rice, it'll leach starch and stick badly to the inside of the pouch. I've found a better way is to use wheat grains. Wet the pouch so it's soft and floppy, open the mouth and pour wheat in till it reaches the top. Tap the wheat in with a bit of dowel, add more wheat. Keep doing that until the pouch has bulged as much as you want it to. Set aside and let it dry.
When dry, tip out the wheat. There'll still be a coating of grains stuck to the insides, so, take a few ball bearings or metal nuts of a size that'll fit in the mouth easily and add them. Put your thumb over the opening and shake the pouch vigorously so the bearings/nuts dislodge the grains from the pouch walls. Tip all of it out, put the nuts back in and repeat until all you have coming out is the nuts/ball bearings.
Don't forget to plug the mouth with something of the right diameter (dowel or similar) while drying so it'll keep a nice round shape.
Eric