Stone, Brick, Concrete. Which material is the heaviest per cubic inch?
That kind of information is easy to find with google.  Here's one of the first that came up for me:
http://www.rfcafe.com/references/general/density-building-materials.htm

Concrete - 150 lbs/ft^3 

Brick - 120 lbs/ft^3 

Crushed stone - 100 lbs/ft^3 

Limestone - 171 lbs/ft^3

If you really want some weight, go with cast iron - 450 lbs/ft^3

You're diagram shows 100 in^3 which is 0.058 ft^3

Concrete will be the easiest to work with in this application, but would come to just 8.7 lbs  A bit more then a gallon of water (8.3 lbs)