Can you add a link to the retropack? :)
Also, where's your Mac build? :)
So part of my Make-A-Thing project will be building an arcade controller mounted on a waist-high cabinet. My plan is to treat this as a controller that I'll then make a game for, based on the Make-A-Thing words. When complete, the controller should then just work as a USB controller.
If you want to incorporate this into whatever you end up doing for Make-A-Thing, what I can tell you right now is it'll be a 1-2 player setup, with each player having an eight-way joystick and six buttons. Let me know if you plan to use this, and I'll loop you in on further developments/complications.