Styrofoam, cotton, multiple layers of paper tissue or paper towels, anything that has lots of air trapped inside. Air trapped in something is a very poor conductor of heat and thus it is a good insulator. Wool is a good insulator. Silica gel conducts heat very poorly, but it might be hard to get enough of it to be useful (it's in those little "do not eat" packs that you sometimes see in foods and medications to keep moisture from building up). Rubber is a good insulator, too (natural or silicone). Glass is a good insulator. Whatever you use, though, make sure there's lots of air in there.
If there's a lamp shining down on it as a source of heat, consider putting two layers of aluminum foil around the insulation. The top layer (closest to the lamp) should have the shiny side up. The bottom layer should also have the shiny side up (towards the insulation). The aluminum reflects radiant heat, such as infrared, so that it can't be absorbed by the insulation. You can use more than two layers with insulation between them if you want. Make sure each layer is independent; don't allow two layers of aluminum foil to touch at any point.
The insulation should be kept light and fluffy, not packed down. This increases the amount of air inside the insulation, which improves its qualities. If you don't put aluminum foil on the top of the insulation to reflect infrared, the next best thing is something that is very bright white. Don't use anything dark on the top, as it will just heat up super fast.