For instance, I got an Evo 3D. I wanted an extended battery to go with it. The only one with a case available is by Sedio. It's super expensive, and I don't like the design. I went about finding a cheaper one; of course, the cheaper extended batteries don't have cases.
Since, I have a FDM printer lying around, I went about creating a custom case for it.
This instructable will teach you how to model the case to fit the phone/any device.
Step 1: Tools and material
* Vector based graphics software (autoCad, Adobe Illustrator, solidworks, what have you)
* 3D modelling softare (Solidworks, autoCad, 3D studio max, etc)