Try cutting out a circular patch from the same or similar material, overfill the valve with silicone and place the patch over and place a weight for a few hours over this. should do the trick.
The patch will prevent any media from disturbing the seal. You should wipe the excess silicone from around the patch when it is weighted. You can fill after 15-20 minutes, provided you place the media carefully so as not to move the patch and upset the seal from curing in place.