Modeling things to fit real-life objects is one of the best and most common applications of 3D printing, and Blender is capable of this. Blender may never be able to compete with other CAD programs for precision; nevertheless, with some careful planning and clever manipulation, Blender is capable of extremely precise modeling.

Also, leaving modifiers unapplied is a technique that allows a high degree of flexibility, which is particularly desirable for something that stands a good chance of being customized. If someone else wanted their own SD card holder ring, it would be a simple matter to resize the ring and adjust to position the SD holder.

Now that you have the knowledge and skills to measure and design in Blender, the world can be at your fingertips.