I created and develop a one-handed system called ARTSEY.io.
Artsey is a one-handed keyboard system using 8 keys in a 2x4 layout. The system is designed to be easy to learn, comfortable to use, and fast enough for practical use.
Artsey uses key combinations to type most of the letters, numbers, symbols, and functions available on a full-sized keyboard.
It is for people of all abilities and for any application. I personally use it to take notes while I walk or for a pocketable hardware keyboard to use with my phone.