How do I make a square hole in a cube, and then save it as one mesh? I need to design a thick slit, in which i will place a lever, kind of like a large power switch that mad scientists have. I was wondering how to make the slit, and save as one mesh. Would really help if someone could tell me how to make the lever as well.
In edit mode, select the top face of your cube. 
Press I and resize the new face till it is the size you need.
Press E to extrude the hole inward.
Press Delete or X and choose Faces to delete the bottom face of your hole.