I want to use my Android Pixel tablet as a second screen for my Macbook Air, similar to how users of the app Duet use their iPad as a second screen for the Macbook. Is there any free/inexpensive app that does this?
I am not in anyway affiliated with the following company / developer but I think iDisplay might be the solution you're looking for. They have an application for Android as well.
I just bougth iDisplay, it connects to your app via ip/network connectivity. This will require your tablet to be on the same network as your Mac.
Think about it when you're trying to use it at work - if your work has policies on that kind of thing.