I went with a cleaning kit from Armalite to plug the hole and have something to clean the rifle some in the field.
M-15™ 5.56 Otis Pistol Grip Cleaning Kit - Armalite
It really depends on what you are looking to store in the grip. I also got a Magpul STR butt stock for additional on the rifle storage. Batteries, broken shell extractor, ballistic tables, whatever.
It depends on what money you have and what you are willing to spend.
As far as the plug, o-rings might work. Maybe a rubber band? I've noticed with the cleaning kits, that different grips have different sized holes depending on the manufacturer. One grip, the kit would not fit and in a couple others, it fit just fine.