This is a Hoffman packing puzzle.
https://en.wikipedia.org/wiki/Hoffman%27s_packing_puzzle
It is puzzle where you (try to) pack 27 cuboids with 3 different side dimensions into a cube with equal sides of the sum of the cuboid dimensions.

A parametric fusion file is included, so it can be made to whatever size. Theoretically there should always be a solution for the 3 dimensions you choose (evil grin).
Keep in mind that for an interesting puzzle, the 3 sides of the cuboids must each have a different size and the sum of the sides must be less then 4 times the smallest size (x+y+z < 4*min(x,y,z)). It will still be solvable, but it will be easier.

I added 0.2 mm play in the design, so scaling is not recommended. Just change the parameters in the Fusion file if you want to scale the design. I posted a slightly smaller version here: https://www.thingiverse.com/thing:4753370

If you are making this as a gift, the solution is on the wikipage, so you can package it.

You need to print the cube, the lid and 27 cuboids:
-1 x 'Cube.stl'
-1 x 'Cube lid.stl'
-27 x 'Cuboid.stl' or 3 x '9Cuboids.stl'
I printed them in 3 batches of 9 in different colors, hence the stl with 9 cuboids.
There is also a single stl file with all parts if you have a big build plate.

If you are looking for a similar more challenging puzzle, check out: https://www.thingiverse.com/thing:4754202
which is the Knuth version (it has 28 instead of 27 cuboids).

Creality

Ender 5

No

No

0.2

20%

I printed the outside walls extra slow for dimensional accuracy.

The play in the parts was fine, but the blocks tend to grip when the print layers rub over each other in the not parallel direction.
I took all blocks and ground then on an old whetstone for a few seconds. This makes the sides slightly dull, but they now slide the same in all directions. Making the puzzle more enjoyable.

Use water to keep the material cool, if you do not, the grit of the sandpaper/stone will get into the plastic and stain it. By rubbing it you heat and soften the PLA and dirt gets trapped in it. With water it stays cold and it stays clean (although slightly matte finish).