There are some little plastic adhesive covers that allow you to easily slide the cover on and off the webcam. These can definitely crack the macbooks. However, tape is fine. I actually bought black masking tape for use on my computers so it's less noticeable. I also put a piece of paper over the adhesive that touches the webcam so it's extra opaque and doesn't get sticky stuff on my webcam.

I used to not care very much about covering my webcam. After seeing Zuckerberg cover his, I do it religiously. If a tech billionaire doesn't trust his very well funded IT staff to secure his laptop, then what hope do I have?