I don't think any kind of passive heat sink is going to work considering the location of the DIS.
With the ideas presented here, you'll just delay how longs it takes for the DIS to get hot..... everything is going to get heat soaked anyway, and you'll reach the threshold that the aluminum can dissipate heat, and in fact you could cause more problems because the aluminum is going to hold that heat, and it has no where to release to other than the ambient air temperature which is hot once the engine is at running temperatures. Also you have to remember that heat rises, and considering you have an insulator on the stock hood that heat is sitting right up there with the DIS.
For a heat sink to work properly, you need a source of coolant be it air or a fluid that is cooler than what the device is so the heat will travel into that medium. Either the DIS is going to have to be moved, or you need to supply at least some cold air so the heat from the aluminum can shed off its heat to.
Also, a flat peace of aluminum is not going to be that good, considering the surface area and the thickness of the material your using.... you need to increase the surface area by using fins etc.
Just my 0.002 cents worth.
Frit