I was thinking about using some bird shot in small 1/2-oz baggies and adding them incrementally. I was going to glue the baggies to the bottom just behind the front glacis. I haven't done it yet, though.

As far as how much, do a search for swathdiver's M41 build. I think that he recommended 8 oz for stock plastic everything and 16-oz of weight for metal everything.