The ZAMAK #3 meets all the requirements for thermal conductivity and mechanical properties.  

But the density of zinc is 2.4X the density of aluminum. The weight penalty is too high, compared to the target density of 3.0 g/cc.  

 The per-pound metal cost of the zinc is lower than aluminum. However, with higher zinc density, the block has 2.4X the weight of the equal volume of aluminum. So zinc for the block actually has higher actual metal cost (3.6 pounds x $0.69/pound), than the block in aluminum (1.5 pounds x $0.93/pound).