I would not use a rubber washer or and O ring. When you shoot the gun and work the action it will work the material until it breaks and then you are back to square one. It might work great for a safe queen only hand cycled, perfect if that is all you plan on doing with it other than taking a few pics.
I would get some thin copper wire and wrap it around where the nut you screw onto the action to secure the foregrip meets up. It will be tight and you can use as much as you need. It will also work forever and never need to be replaced.
