> I wonder if a whole external device is really necessary for the one-handed device for chords. I think with a customisable keyboard, you'd be able to chord.

Not with a normal keyboard (though there do exist specialized keyboards for gamers with which this might be possible). The reason lies in the key rollover, which is very limited for many off-the-shelf keyboards:

> https://en.wikipedia.org/w/index.php?title=Key_rollover&oldi...

> https://en.wikipedia.org/w/index.php?title=Key_rollover&oldi...

"To reduce cost and design complexity, most computer keyboards do not isolate all keys. Instead, they use a matrix of key switches, without any isolation diodes, that assumes that only a limited number of keys will be held down at any given time. With these keyboards, pressing as few as three keys can cause ghosting effects, although care is taken when laying out the matrix arrangement that this does not happen for common modifier key combinations."