ok im getting all my speakers installed soon, there powered by the headunit

more importantly, ive bought the ed nine 2x and the 12" 600W RMS sub they offer.
installers have been saying to build a 1.5cubic foot box with a port for my rap music in the trunk stretching from the carpeted wheel well to wheel well.

the problem is,
is it okay to mount the amp on top of the box? two installers say yes (if they vent the "rear dash") and one says no, bolt it to the side of the box because it would be too much heat.
what do you suggest? it may be a little tight up there, but how would it be?