You can buy those at home depot/lowes. It looks like a large soffit vent. I ordered a couple while I was building an enclosure for my tegu. They come in different sizes, you just have to make sure which raduis/style you need.
It is a mesh insert for the top of a wooden vivarium for you to sit your basking dome on, because at the minute my basking dome is inside of the vivarium this has lead to minor hand burns on my panther and also means that most of his furniture cannot be near it or as high up for him as he needs.
The picture I took is of one on a vivexotic viva tank, they come with one supplied but I am convinced I have seem these on sale separately long long before the new vivexotic tanks came out...but now I need one I cannot find one