Update:

This model seems to be harder to solve than I originally anticipated. I posted this as a challenge, not providing the solution along with it. This seems to be causing a lot of controversy (Check the comment section). I decided to disclose the solution to this within one year of the release, which is around September 4, 2026. 
(Only if nobody gets it until then.) 
Up until then, you may check the comments if you are willing to give this model a shot.

A  3D puzzle where you fit 12  pieces into a 5x5x5 box. 

It is as simple as that!

Your task is to fit 6 C-shaped pieces and 6 I-shaped pieces into the box.

That is no overlap - nothing should stick out!
It is called Shipper's Dilemma.
 

Here a general description of this sort of puzzle:

"Shipper's Dilemma" is the name of a type of difficult wooden packing puzzle. The objective is to fit a number of uniquely shaped pieces, typically made of wood, into a specific container, often a box, to form a perfect cube or another solid shape.

The most well-known version of this puzzle, sometimes called "Conway's Packing Box," was developed by the mathematician John Horton Conway. It's designed to be so difficult that a solution cannot be found through random trial and error. The solution requires a high degree of analytical thinking and spatial reasoning. There are various versions of the puzzle, with different numbers and shapes of pieces, such as the "Y" and "Z" versions.