I was thinking of things I could model and 3D print, and for some reason, a miniature filing cabinet for SD cards popped into my head. It would act as a normal filing cabinet (miniature, of course. About 8 inches high at the most), with drawers, but each drawer would have five slots to insert SD cards. The advantage to having this over a little case, is that each drawer could hold different things. For example, one drawer would be marked with Family Photos, another Business Files, and another Raspberry Pi (totally!). The downside, is that people use USB flash drives for files more then SD cards. I would create this for USB drives, but they can come in all kinds of sizes with their cases. Now that I think about it, there isn't any downside to using SD cards over USB flash drives.
I drew up some plans for this (I never knew a ruler could come in so much use), with the exact inch sizes. Then, I gave this to my brother who uses SolidWorks, a program for modeling but especially for things such as 3D printing and working with physics. You can see his blog post about SolidWorks on his blog here. Here is just a single paper that I worked on:
This paper shows different sizes of the SD card, and the drawer. Here is a GIF I made in Blender of the drawer (haven't yet created the full cabinet):