Step 12: Rubber
To simulate these I've chosen to make slipcast latex pieces. The process of slipcasting latex is pretty straighforward. I learned everything about it from the tutorials available at monstermakers.com which, incidentally, is also where I buy all of my latex slipcasting materials. This is essentially the same process used to make latex Halloween masks.
The short version: sculpt whatever shape you need using oil-based clay, make a mold using Ultracal-30 or an equivalent gypsum moldmaking compound, remove the clay original, fill the mold with liquid mask making latex, drain the latex back out of the mold, and wait for the remaining residue to dry inside the mold. When the latex is dry, you can peel it out of the mold and you've got a skin shaped just like the original clay sculpture. If you need a thicker skin, you just repeat the pouring and draining process after each layer has dried.
I used this same process to create the rubber bits for the inside of the elbow, the back of the knee, the hip joints, and the neck.