I used "GREAT STUFF" insulating foam sealant, for gaps and cracks.
I used it on my TM VSR G-Spec and it cut the vibration and rattle, almost 100%. The only sound I have coming out of my gun, is out of the barrel...
My TM is different than your L96. All I had to do, was put a plug in the trigger mount and fill the stock little by little. I filled the gap 1/3 and would let it cure for 8 hrs...
Make sure you mask and plug any areas that you dont want foamed. It is very sticky ( Similar to that of LOCTITE ) and can make a mess of your guns exterior.
I filled all and any vacant cavities, making sure that it will never interfere with the action of the gun.
This is a great/cheap mod...
Have fun