Even if you don't own a 3D printer, designing for one is handy. Plus, you can always ship your designs off to a friend, or service that does have such a device. You should eventually get one though, and they are relatively inexpensive nowadays.
One of the beauties of Computer Aided Design (CAD) is that you don't actually have to use CAD software to create objects in the real world. Confused? Don't be. CAD design and 3D animation software use very similar methodologies for modeling objects. They even share (some) file formats. And any 3D modeling software (worth its salt) can import and export in CAD formats.
You don't have to spend a boatload on AutoCAD or SolidWorks to create and prototype your designs. These programs can cost thousands of dollars (per year, or even per month) to own. Instead, you can use free software such as Blender to model your objects. The additional benefit to these easily accessed software packages is that there are tons of YouTube...