I call it "Wubber". ("Wilicone" was quickly rejected!)
Mix up some lead shot (I buy mine from Hunting, Shooting and Fishing shops) with some silicone rubber. You've then got a stiff goo which can be packed inside the hull. It has a little flex when set but will not shift.
I've used resin for this in the past but resin is runny and smells and sets too hard. (Not suitable for the near-organic U uPVC class!)