Your dilemma is not that uncommon, but trying to find the right path is always difficult. You must first define certain aspects of the prototype you’re creating. Is your prototype eventually going to be created out of what material? Wood, metal, plastic, or what? Then are you going to require more than one?
Modeling clay is probably the easiest way to sculpt out your prototype. But you also have to consider the shrinkage that occurs when clay dries out. If you don’t have a kiln, then find a ceramic shop in your area and pay them to have your project fired.
DAS clay is considered earthen clay, which means that it is collected from known clay deposits and refined for use as sculpting clay. Here are the definitions on the different types of clay.
The professionals that create prototypes for major manufactures use different types than what you’re using. You can go here and find the types they use.
Here again you must make a decision on how many prototypes or copies do you need. If you require more than one, and the final product is either made out of plastic or metal. I suggest that you create it first out of clay (not dried or fired to maintain shape and size), and then create a silicon rubber mold. Then you can cast plastic resin or a soft metal like (Babbitt or lead) which can be melted on a propane stove or natural gas stove.
If your prototype is of a mechanical nature, then it would be wise to have it machined from machinable wax or wood first, then have several made out of the type of metal you require, once you have refined your specs.
You may want to take a look at some videos of DIY mold making.
If you would like; you can send me a PM and I will help you in your choice as to material and procedure as to which would be the best! I can send you a non-disclosure agreement if you wish (to protect you not me).