I think a piece is missing from this puzzle. In your photo you have 18 pieces, in the folder there are 17. (no file "e.stl")

Sorry I gave my cube away so cannot post. To make it easier for them I painted each side a different color. Still hard for them. I had to print the solid cube and tried to follow. Even then I realized I had a space in the center. Finally found I could swap two pieces and it worked. Found a pic but poor. Marked off solid cube. Need to print a better group of pieces.

First off, the file is complete. While placing the various parts into my slicer program I managed to mirror one and it was causing A LOT of confusion for me. All in all, a very challenging puzzle, even with the "solution". Only advice I can provide is to keep changing out pieces even if they fit as there are a few that can be placed and match the outline but be incorrectly placed.

Added a name extrusion and pen holder to compose a much appreciated gift. Thanks for the puzzle. I added a blueprint to the stand as it was too difficult without any hint. I wish there was a clear bottom to build up. A lot of the difficulty was to not crumble it all up while solving. 0.12mm layer Red Copper generic PLA from Banggood 3dfila white pearl

Great model. Pretty hard to solve. Thanks!

Solution cube might help but we've (my son and me) managed to put the pieces together using "brute force". :-)