He says its an “aluminized metal”. Looks kind of like the Aluminum-look acrylics from several suppliers. Could be thin steel with a painted or anodized aluminum color.
I posted a photo of some sample material I got (here: Where to get good materials for laser projects?) that was not aluminized metal, but metal-look poly products, some of which came with adhesive on the back all ready. More similar products on inventables https://www.inventables.com/categories/materials/acrylic?utf8=✓&selected_filters[Effect][]=Metallic