I use a keyboard called PentiKeyboard. It's a chorded keyboard. It allows you to place 6 buttons (shown in outlines) overlaid on top of the whole screen.

It had 2 features I wanted to remove. If you swipe in empty space, you can use it to adjust the screen brightness. I kept triggering this accidentally and each time it disabled auto-dimming on the system. And the other feature is that when the keyboard is open, the volume buttons are repurposed, I wanted them to remain working normally.