Weights and lead shot -- I started thinking (always dangerous).  Smallest shot packs more densely.  It follows that lead powder would be densest of all.

Turns out that lead  powder is used to add weight to golf clubs, and is available (except, it seems, in eco-freaky California)  I bought a  pound of it for about ten bucks.

Drip CA or matte medium into the cavity, and add lead powder.  Works.

Of course, for things like flat cars, sheet lead, 1/16" thick, is sold for roofing purposes,  Cut to shape.  Use two layers if necessary and possicle.

Also useful for replacing or auglenting the weights in any kind of car.

Peter Weiglin