Clear plastic sheets can be found with varying thicknesses but for the purpose of a greenhouse experiment, they should be fairly thin. Using any container large enough to hold a thermometer, secure the plastic sheet over it using duct tape. Make sure that there are no leaks that will allow heat to escape. Because the plastic is clear, it will allow in UV rays, which can raise the temperature. However, it may not trap heat as well as other materials because it is so thin.
Glass sheets can be used that are either thin or thick. The glass will let in UV light which can raise the temperature but it also is not a great insulating material. This means that much of the heat that enters the container may escape. Glass is a great way to mimic smaller amounts of greenhouse gases in the atmosphere because it will allow a fair amount of heat to escape.
Cardboard is a fairly decent insulator that will trap a good amount of heat. This can mimic an atmosphere that has quite a lot of greenhouse gases present, which hold heat in and warm up the Earth. Dark cardboard will heat up faster than lighter colored cardboard and can mimic an even greater amount of greenhouse gases. Using a container that has clear sides such as an aquarium is an excellent device to use when using cardboard as your cover.
Carpet is an excellent insulator and has the ability to mimic a large amount of greenhouse gases. Secure the carpet to the top of the container with the soft end facing down. Different types of carpet can be used of varying thicknesses to imitate different levels of greenhouse gases. Make sure to use a container that has clear plastic or glass sides so that UV rays can enter it and warm it up. Even though greenhouse gases hold heat in, they still allow UV rays to enter the Earth. Carpet will block those UV rays so you will need to ensure that they have a means to enter the container on the sides.