Hi Mike,
I decided to use aluminium, 1.6mm. I did a fair bit of reading on thermal conductivity and heat radiation, and from what I read aluminium will reflect heat better than stainless steel, and the "mirror" effect on some stainless does nothing to aid in heat reflection. If the shield was actually touching the hot parts (exhaust) then aluminium would not be good as it conducts physical transfer heat hugely, but with the air gap you are relying on radiation reflection and aluminium wins out there. I will discuss with my powdercoater if heat would be an issue on the finish as I want to powdercoat them, otherwise maybe anodize.
For bending I bought a 450mm brake press on ebay ages ago, makes lite work of aluminium up to 2mm, 1.6mm stainless is a chore but do-able, you wouldn't want to go any thicker with this thing. The bender looks exactly like this one on fleabay;
450mm X 1 5mm Steel Sheet Metal Folder Bender Bending Brake | eBay