This video shows you how to make a flexible mold using Amazing Molding Putty. You can make molds of just about anything that has the desired shape and texture you want.
The putty comes in two parts that are kneaded together. Take a piece of each part that is equal to about half the size of the thing you plan to mold. Make sure you have the object you plan to mold ready because after you combine the two parts, you have a limited tome to press the mold.
Combine both parts of the putty and knead them together until you achieve a uniform color. Form the putty to the shape you need and press the object into the it. Press down firmly enough to make the putty rise above the edge of the object. Let it dry and you have created a flexible mold.