It is often the kind of thing that is hardly ever worth making, unless you have the equipment and the skills. As others have said, you will often spend more on the part than on new fixtures, especially if you add in your time to find it or make it.
Having said that, I'll also say that I've often made parts to replace a variety of things from plastic, from the handle on a well used kitchen pot to rebuilding the wheels and axel of a hand cart. It CAN be an interesting and fun problem to solve.
For your problem, I'd not be looking for a 3-d printer as others have suggested, but for a translucent thermo-plastic, that with some carefully applied heat and a form of the proper shape can be deformed into the shape you desire. See if you can find a piece of sheet plastic, then build a form. It won't be trivial to do, as it will be easy to apply too much heat, or too little. Perhaps an infra-red bulb or two will give you nicely distributed heat.