Unity spotlight questions
 

I recently bought a '94 9C1 Caprice that has the hole in the driver's side post for a spotlight, but came without one. I am looking into purchasing a used Unity spotlight and installing it, but am looking for correct measurments I will need. I know the stock light from '91 - '96 was an S6 model, but this seems to be a generic model with multiple sizes and lengths. If possible, I would like an upgraded unit (black housing and rubber handle) rather than the original equipment, so if anyone can help me with the needed measurements I appreciate it.