Not sure about the undetected part, but to make box ESP, you MUST be familiar with the Direct3D API.
A very simple method, no reversing skills required:
-Find the strides for the primitives you want to draw a box around.
-DirectX supplies you with a world to screen API, use this to find the coordinates from world space to your screen.
-Draw boxes around the given coordinates using some DirectX API.
Now that you know what to do, start learning the basics of C++, then get familiar with Direct3D. Good luck.
Note: There is no such thing as a "simple" box ESP. Just because it's a single feature, doesn't mean it's a piece of cake.