These models were designed for use in my science classroom. When teaching crystal structures I wanted a model that shows how layers of close-packed atoms can form different crystal structures depending on how the layers are stacked. I also added a model showing the difference between cubic and close-packed atoms.

I started with the laser cut acrylic layers (dxf file). The circle size is similar to a penny so you can stack individual pennies to show the two different possible locations for the next layer. These are nice to show to a large class using an overhead projector or a light box and document camera. However they are tricky to align as they do not have "dips" that force the next layer to sit in the right spot. The 3D printed spheres solve this problem. The dxf file also includes a cubic array (not close packed) so you can easily compare these two arrangements

The STL files A and B are mirror images of an array of half-spheres. Print one each A and B then glue together to make one layer of close-packed spheres.
You will need to make a total of three layers of spheres - or you can make two layers of spheres and have the bottom layer be half-spheres as shown in the photo. Stack two layers. Now add the third layer and look through the model - can you see holes that go all the way through? Your arrangement is ABA which is the way atoms pack in a hexagonal close-packed crystal. Move the top layer over slightly so the spheres fit in the "holes" - you won't be able to see all the way through any more. This arrangement is ABC which is the way atoms pack in face-centered cubic crystals. Print several sets and pass them around the classroom so students can observe the two different packing arrangements for themselves.