I need to create a transparent box that can stay warm inside while in cold temperatures, below 0 degrees, for as long as possible. Glass and plastic are transparent but afaik they don't insulate that well. Is there something else that I could use?
To borrow from a certain Brit physician, "I think it's a bit more complicated than that." For example, you could build/buy a double-walled box with a vacuum between the layers. That's esssentially what a thermos bottle is; there are varieties of drinkware that are built this way. But if you were to build a double-walled glass enclosure that's not evacuated (i.e. easier to build), then fluid dynamics comes into play. While dry air is a pretty good insulator, heat transfer depends both on the thickness of the layer and maintaining a static environment. Too thin, and not much insulating power. But, as manufacturers of storm windows know, if the spacing exceeds about a cm or so, the air starts to form a vortex, moving up on the warm side and down on the cold side. This leads to a lot of conductive heat transfer at the glass-air interface.
Unless you really absolutely positively have to make the entire thing transparent, consider leaving a "peep hole" but covering the rest with a reflective metallic surface.