Thats a very good heatshield mate and a superb price as well, Good man.
I didn't go through the whole thread that you linked (sorry too lazy to read all those pages, lol!) but if you want a good material to insulate from heat then a sheet of neoprene and/or nitrile are very good to glue/bond onto the inside of the heatshield. These can be bought online from quite a few places, just need to google it. It will probably put the cost up a little bit but i'm sure if even people want to buy the sheet themselves and put it on it will be relatively cheap, easy and straight forward.
Hope that helps mate.
Please excuse my ignorance but wont Neoprene and Nitrile insulate and contain the heat inside the heatshield? Like seal skin gloves and wet suits? Rather than reflecting it which is what you want.