Learn latex modeling.
Basically go to a sex toy store and get a "make your own dildo kit." Then make something the rough size and shape of what you're looking for, and follow the directions on the dildo kit for how to make the penis cast, using your new model instead of the penis. After that, you should be fine. It should take about 28 hours if you work diligently, and about 24 of that is waiting around for the new toy to harden.
-M