Portable instrument for transcribing music

I'm in high school and I wanted a portable instrument, preferably a key or string one, so I could take it with me for the purpose of transcribing in my free time, I have a melodica but I've already tried using it in public and it's too noisy for a public environment, plus it goes out of tune over time which is quite inconvenient, I thought about a stylophone but it's expensive and limited, does anyone have any suggestions?