I personally have one underneath my glass vivarium (not inside), taped to the bottom with polystyrene covering it to insulate the heat. I also covered the mesh top of the vivarium with polystyrene too to stop the heat escaping from the top. I had thought about having it on the side but I had the same issue with the foam background and didn’t think it would do enough.
I believe it’s not recommended to put them inside of the tanks unless you have a wooden tank, but I’m not 100% sure.

its a good idea to have a thermostat attached to it too if it didn’t come with one!