What can I use as an insulator?

I have to build a 20cm x 20cm x 20cm cube containing a beaker of waterI'm only allowed to use wood, paper, cardboard, natural fibers, organic granular material, and aluminum foil to retain the heatWhat would you suggest? Would sawdust be a good insulator?

Answer:

I assume you mean cookie sheetI like the disposable foil because I'm lazy at clean up, and can stack of hundreds of dozens of cookies waiting to be baked batch by batch without investing a fortune in my cookie sheetsA heavy NSF-rated aluminum lined with silpat is the best, for me, when I'm not in production line mode of cookie making.

Use Plant-based parchment from the super market on top of any kind of trayYou can even re-use it.

Yes, saw dust would be a very good insulator or shredded news paper is also a very good insulator and is used as such in housesI don't know which is better but don' pack it at all leave it loose.