Puzzle description:

Singmaster Packing is a box packing puzzle where 25 identical bricks of the size 1x3x4 should be packed into a 5x5x12 box. The puzzle was designed by David Singmaster as the simplest non-trivial example for brick packing where the box is not a multiple of the box.

If you can read German: Mehr Informationen gibt es in https://welt-der-geduldspiele.blogspot.com/search?q=3D-Druck

Puzzle difficulty: 

The puzzle is of medium difficulty because all bricks are of the same size.

Printing instructions:

Everything is easy to print. Prints are fine with no supports. 
I printed the puzzle in PLA at 0.1 mm layer height. Just print the box and 25 copies of the brick. Bricks in different colors look more interesting.
The STL file was generated using PuzzleCAD: https://www.thingiverse.com/thing:3198014. The PuzzleCAD .scad file is also included.