Step 1: Designing in CAD
While designing, I simply measured the location of all the important parts that I needed access to ( USB, Ethernet, HDMI, etc.) and made sure to create the proper sized holes in those locations. Using a pair of calipers is much easier than trying to do this with something like a ruler.
I designed the whole case as one piece, then used the split tool to cut it in half. This made it easy to be sure everything lined up. Also, be sure to design in some way to hold your case together. I did not want to need any extra hardware for my case, so I just designed some T shaped nubs that used friction to hold everything in one piece.
I attached both the top and the bottom of my part to the file here in STL format. If you are making your own file, all 3d printers that I have heard of take files in this format. The files I have saved here are in centimeters, so some scaling may be needed depending on what units your 3d printing software uses.
Step 2: Printing the Case
There are services that can print files for you, but it was nice to have a 3d printer available since I had to redesign my part a few times after printing it out. When testing for a fit, It was easiest to just print the bottom half, as that gave me a pretty good idea if the top would fit as well and I did not have to use up another couple hours just waiting for it to print out.
Step 3: Assemble the Case
The two tabs in the top should easily slide into the bottom part of the case. These tabs actually held a little better than I expected. I had to use a screwdriver in order to pry them apart.
My goal was to make something that required no tools or hardware to put together and take apart so that it can easily be done on the fly. I nearly accomplished that, as all that is needed is something thin enough to pry the case apart.
Now all that you need to do is start making cool stuff with your Raspberry Pi!