Speaking from an engineering perspective, why would you ever want to add a larger tank in the engine compartment? Any tank that gives you the chance to ice it is a good thing, but one just for more volume, especially in the engine bay at whatever underhood temps are, is counterproductive because it will absorb more heat and put more load on the HE. If you want to add capacity and make it effective, get a bigger HE. Then it's expelling heat rather than absorbing it. Something in the trunk gives you the opportunity to loose some heat to ambient, but the downside is the extra capacity will take longer to cool down if you experience any heat soak. My opinion, for what it's worth.