I taped and cut newspaper to the inside of my windows and then traced it into the cardboard. It is not perfect at all. If I were to park somewhere dark and have any sort of light on inside, you would easily be able to spot me from light coming out cracks all around the edge of every window. Spending the nights in parking lots is okay though because there are so many lights that reflect on the windows, you'd never be able to tell some are coming from inside. My windows were already pretty tinted, the cardboard is mostly to deter those that walk up and cup their hands around their face and press their nose to the window. I don't know how I'd handle it if I woke up and saw someone doing that.
Also, I have to wake up at 4 or 4:30am for my job, so at 8 or 8:30pm it is still really bright outside, and the cardboard does a great job at keeping it dark inside.
I also started to decorate the insides of the cardboard, and decided to just leave them up all the time. I have a poster taped to one and a map of the park to another. I plan to make them better/prettier. And even eventually make ones that fit perfectly (maybe...)