Going by the photos it seems the engine may be too close to the cowl at the back. Air flowing past both sides of the engine try to meet up but can't so it forms a stagnant area at the back of the engine. Depending on how much room there is at the back of the engine you can try fitting a wedge shaped piece to the firewall so the point ends very close to the fins. This will give air coming either side of the engine a chance to flow around the back but even then it really needs exit slots for the air to come out top and bottom of the cowl.