I know that an electric eraser sounds really lazy, but it is actually useful and fun to make. This is a fine example of chindogu. It erases quickly and neatly, but is a hassle to carry and use and makes you look lazy.
Prepare the box. Place the motor in the box and mark where the shaft is. Drill a hole big enough for the shaft and the first small base to fit into. Using a dremel with sanding attachment, sand off the little vein-thingies so your motor goes in all the way. Place the motor back in and put the battery holder in (with the snap on) and find a place on the box lid where the switch can go without hitting anything. Drill a hole for it and then mount the switch to ensure it fits, then remove everything.
The circuit is really simple. Run the ground (black) wire from the battery pack to either of the motor leads. Twist on and solder or use tape to make a good connection. Run the positive (red) wire from the battery pack to one lead of the switch. Solder or tape. Run a wire from the other lead of the switch to the other lead of the motor. Press the button and the motor should spin! If not, check all the connections and make sure the batteries still have some juice. Simple, huh?
Mount the switch in the hole you drilled earlier. Put the motor through the hole and use hot glue (I actually used E-6000 glue) to secure it in place. Before you put in the battery pack, put a piece of tape over the metal tab on the end of the battery pack without the snap connector. Now squeeze it in the box. It's a REALLY tight fit. When putting the top on the box, be gentle. The motor is fairly large- large enough that I could not shut the box all the way. I just screwed it in until I felt like I was about the break the box.
Poke a small hole in one end of the eraser. Put super-glue on the motor shaft and then push the eraser on.
Go erase stuff! Push the button to get the eraser spinning, and go erase stuff like a real geek!