Molds and Custom Composite Parts
Many parts in rocketry (and other hobbys) can be produced yourself out of composites (and sometimes plastics or other materials), such as nosecones, transitions, fins, and custom detailing pieces, among others.The process starts with a 'plug', or the piece you would like to duplicate. Then, you create a negative around it, of which is usually two parts, sometimes more. This negative is then used to recreate the part with composites, effectively making an exact copy of the item (however, your resulting product may be stronger, lighter, and easier to fix or replace - all of which are beneficial in our hobby!)
Many complex molds commonly use silicone molds and plastic or resin compounds in the cast part - this is a perfectly acceptable way to make parts, even in rocketry, and I have done it before succesfully; however, I will not go into using these materials (at least yet). I prefer composite molds due to their cost, materials I have available, stiffness, and overall reproducability and accuracy.
All you need to get started is a plug (the part to duplicate - i.e. nosecone etc.), composite cloth/mat (fiberglass, carbon, kevlar, etc.), laminating epoxy, PVA release film, mold release wax, and parting board materials. I would strongly urge a confidence in working with composites before attempting making molds and parts. I would suggest trying to glass a few body tubes first to gain experience with the materials and general tips and precautions before moving on to more advanced topics.