A simple print to have some stackable shelves. I have designed these so the .scad file is parameterised to fit any different size that is necessary, but be aware that if you make the height or depth too low, the hexaganol pattern rows/columns will need adjusting.

Simply print the shelving, slot the pieces together with a small amount of glue, although it's a tight fit and shouldn't really need it, then add glue to the top of the legs and stack another set of shelves on top if you so wish.