Looking at your pic, it seems clear that the vent holes are not at the highest part of the case, when the Mac is mounted at that angle. Would propping it forward such that the vent is at the top help?

Another thing - after blowing one power supply in an iMac 450, I made sure there was enough space under the Mac for cool air to get in as well. I used the front fold out foot, and a couple of small blocks of wood at the rear.

THere's always the olde MacChimney approach, too.