The easiest material to use for this kind of project is silicone putty. This is a common material used for making molds. The brand that I am using is "Silicone Plastique." It can be purchased from MakeYourOwnMolds.com
. Other silicone putties can also work. Just make sure that it is rated as "Food Grade."
Silicone putty typically comes in two parts (Part A and Part B). Each one by itself is soft and has the consistency of Play Doh. But when they are combined, they quickly cure and harden.
Before working with any silicone putty, you should read the material safety data sheet. This should let you know if there are any health hazards associated with this product and what safety precautions you should take. In most cases you will at least want to wear gloves while working with it. Also keep in mind that the silicone putty can stain some work surfaces.