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)
Step 2: Preparing reference photos
The way I accomplish this (since I don't have a 3D scanner) is to create reference photos of the object's projections. I used a scanner to get exact orthogonal views, however, if you're handy with a camera, you can use a camera instead. I suggest trying to fix the focal point of the image to be at the same spot for all views to keep it consistent.
Resize the photos outside of the vector editor if you want, I did it in Adobe Illustrator.
To get the sizing correct, choose as many reference features in the images as possible (the size of the camera button, the size of the camera lens, the length of the body, etc). Use calipers and rules to measure those features, and adjust the scale of the image accordingly.
Step 3: Drafting the case
Layout the reference images in the vector graphics editor of your choice, layering it, grouping it, etc in the way you like.
Use guide rules to get the size correct. I would double check the size of the reference images here, and make minute scale adjustments.
For more complex case designs, I'd sketch the case first, but since this is a simple case, and I like to design on-the-fly, I winged it.
* When designing a case, think about how the device locks onto the case. If it's a two-part case, think about how the halves are held together
* including features in more views is a good precaution.
* If you're using a non-cad 3D modelling software like 3D studio max, all of the draft images are merely reference images. You may use them to extrude features, etc, but they're not hard requirements as you can manipulate the faces of the 3D model directly
* If you're using a CAD based 3D modelling software like Solidworks, the draft is important as you are using the contours and features of the draft to construct the model
** a brief overview of how it works,if you haven't used it before,
In my case design, the case holds onto the cellphone with hooks/snaps that grabs the bezel of the screen.
Step 4: 3D-ify the draft
Copy and paste or re-import the draft for each side you have. (I like to center things around the origin)
After setting up the draft, extrude, revolve, loft, extrude cut, fillet, chamfer, what have you to built out the 3D model.
Step 5: Prototype the case
If you don't have a 3D printer, you have the option of getting it printed elsewhere, like http://www.shapeways.com/
Once you have the print, you might need to alpha test it, make modifications to your design, and then reprinting.
Sometimes, it takes a few iterations before you can get it right.
For instance, this case took 3 iterations to get correct. I also made a kickstand version of this case which took another 5 iterations to get the kickstand mechanism working.
Step 6: Share/open your work for others.
If you're not planning on making money off of your design, consider sharing your work. You can adjust the licensing such that others can't make a profit off of your design.
You can find both of my cases on thingiverse.
For those who use models from thingiverse, consider flattr-ing the creator. They are sharing it for free without expecting compensation.