I use the Microsoft Bluetooth Mobile Keyboard 6000 [0] for portability. It lays over my laptop keyboard without changing the profile significantly, and it fits into any bag virtually unnoticeably. It's honestly one of my favorite non-mechanical keyboards. The trouble is finding one at a reasonable price, which is why I limit its use to while on laptop-only.

[0] https://www.amazon.com/gp/product/B002NP8XJ0