The quickest way I've found. Consist in using fishing weights made of lead and zinc.
First put a little of hot glue to prevent a rattle. Add weight, interspersed with hot glue. Press to stick properly.
Finally, cover the bottom with felt fabric.
MODEL STAUNTON - MASSIVE POLYETHYLENE HIGH IMPACT
KING WITH 10.7 CM
APPROXIMATE WEIGHT 1.7 KG