In this case I do not really think you are doing anything that will create a noticable difference. I do not have a Duratech, but looking at them, it looks impossible to create a heatshield that would not defeat the purpose of an open air element. Zetecs are certainly different.
If I were you I would focus less on trying to slow down the heating of your air filter and more on trying to remove heat from the air filter. But what do I know?
"A true heatshield should cover the whole filter."
Nope, a true heatshield should shield it from heat. If something on one side of the air filter has less heat than the air filter itself, you certainly do not want to shield the filter from heat.
