He doesn't want a stenotype keyboard, though, he wants software that lets him use a regular keyboard
as a stenotype keyboard.
So, based on Wikipedia's image of a steno keyboard, and one other image of one...

Number bar is in yellow. Most of the keys on that layout actually won't be useful, but I left them in because they have system control functions as well. (Yes, even backspace. Ctrl-Alt-Backspace on X.org. Although, I took out tilde, and that's got system control functions on Mac OS, but I digress. This wouldn't be a remap anyway.)
The asterisk key (which is used for backspace on that layout) is double-wide so that the home keys can be used as the stenotype home keys.
There are more concessions that could be made to take advantage of the QWERTY layout's key positioning (such as moving the number bar down to the spacebar, or even just using the QWERTY number row for... numbers...,) but they wouldn't be accurate to the stenotype layout.
And, obviously, this would
demand an N-key rollover board - at least 6-key, but you might need more than that.