James,
When you look at the weights that are available from several sources they are 1/2" squares that fit into the 1" boxes with a center bolt. The tip weight boxes are available from Brodak or Tom Morris assembled or parts. Since the box itself weights about 1/3 to 1/2 oz (depends if it is lite ply or Birch ply) you don't normally need to much more for stunt.
Best, DennisT