Base with the top opened up to insert the rubber gromit.  
The Bulb pops in and out, making it easy to replace, and preventing light from leaking out.  

Always test your wiring!

(Switch added)

  
Before gluing the cauldron in place. Because the cauldron didn't fit flat, I opted for this route. I thought it was the best way to keep light from shining out in between the cauldron and the base.  
 
  Click here to see the finished base. I added a wooden extention to make it easier to pull the bulb out, and attached the switch.