I remove endcap, fill a bridge across with liquid silicone about 1 inch
from end to stop weights moving forward into cavity.
Then use small fishing weights until I reach required feel.
Once correct weight is determined I use some more liquid silicone,
just to glue the lead pieces together and secure them against the
other silicone which will have hardened
out by then. Replace endcap and grip and nothing will come loose,
no matter how hard you hit that ball. The weights will in an absolutely
permanent position, and the silicone may also help absorbing
vibration.