I agree....copper would be the best thing for that purpose (thermal decoy).
Why? Not because it "holds heat in" ,but because it will absorb a whole heck of a lot more heat than aluminum.
Copper is much more dense.
Yes aluminum would cool off quicker. One big reason is that it DID NOT ABSORB AS MUCH HEAT.
Another reason is that Aluminum will radiate heat slightly better than copper. This doesn't have as big of an effect as you think.
We are not talking about blocks of metal that have been heated before hand to serve as decoys.
We are not talking about car parts where strength and weight are a concern.
We are talking about metal blocks that have a constant heat source (CPU) and forced air conduction cooling (fan).
In this situation I still feel that copper is superior.
You want to conduct heat away from your CPU.
Copper is a better conductor of heat.
C U L8R