I use a piece of tape but they make little sliding plastic covers for the front facing camera that is used by webcam apps.
https://www.amazon.com/Nano-Shield-Desktop-Macboook-Computer-Smartphone/dp/B073YSJDFH
That’s not really for Alexa, it’s for stuff like this: https://www.usatoday.com/story/tech/columnist/saltzman/2018/03/01/has-someone-hacked-your-webcam-heres-how-stop-cyber-snoopers/377676002/
However, I have disabled Alexa on my kindles, and I deny microphone permission to most apps on my phone, unless I really use that feature.