it doesnt have to be stainless or even aluminum. i use galv sheet metal, and it can be as thin as 24 guage. can buy that in a sheet from many locations, pretty cheap. even home depot has them in smaller sizes which can be overlapped. i did this once in a pinch.
more recently i've used 1/8" steel i bought from surplus in 4x8 sheet which i cut down with the sawzall. and plan to paint it brick red with the mortar lines on it for that fake brick look lol. but 1/8" it way too thick for whats neded.
the 1" space off the floor is key, allows cool air to go it and therefore heat get out the top of that 1" gap behind the shield.
here is a good general guide i found: