I suspect it is DiBond:
http://www.alucobond.com.au/products/dibond
Not sure about the US but in Canada the mirror finish is not a typical stock item. I buy lots of the white and brushed sheets at my sign shop. I didn't use it on my car because it is quite rigid, and for many of my shields I wanted some flex.
Also, the aluminium mirror surface will not be as durable as the stainless and once marred it will be that way forever.
It sure looks awesome now though! Nice work!
Mark
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