Michael;

That's OK, because every bit helps, but I really think you would get much more thermal advantage from a piece of sheetmetal in the back, if this blocked hot air from exhaust downpipe from getting into Alt Cooling Air Intake (which is at the rear, see below), and maybe directed cooler air from the side/front of Alt. This would obviously need to well stay clear of the electrical connections back there...including under thermal expansion/vibration.

I guess what I'm saying is: Radiation thermal transfer will more than be negated by forced air-flow, so an airflow director will always outperform a radiation shield.

Source: http://www.sw-em.com/altkit_additional.htm#Alternator_Thermal_Considerations
Cheers