Bubble wrap is not acceptable when you want to see through the window. I bought a new mattress a few months ago. It came wrapped in thick, clear plastic. If you wanted a free window covering, that plastic would work and let you see out of it. You’d need to pull it firm to maintain that air gap – and for maximum visual clarity too. If it’s warm to touch, it’s working.
I didn’t use mattress plastic though. I paid for the genuine product. What you get is a thin, transparent film and tape. The film is high-tech, ’cause after you put it up you play heat from a hair drier or fan heater over it and it tightens. If you’ve done a good job in putting it up, the result is almost invisible and definitely see-through.
My system of lining my windows with foam first caused some problems when I tightened the film. The foam is flexible, so it pulled in, and pulled away from the window, once the film tightened. I had to modify my system by using wide, clear tape on the foam lining before sticking on the film. Then I used extra wide, clear tape on top of the film to stop it pulling away. And finally, I used the heat sparingly, trying to keep the tension on the film to a minimum. I think it would have been better to frame my windows with something stiff, like wood, but I’m not a carpenter and I had the tape.
To finish, I strongly recommend window insulation for every room in the house. Start with those rooms that get winter sun. When those rooms are insulated during the day, the winter sun warms the room up and it stays warm. The difference can be really dramatic. When it’s 7 or 8 °C outside on a sunny day, my study gets to 27 °C or more quite regularly with no extra heating. And it stays warm in the evening without any additional heating.