Yea, that's actually a great point!
I glossed over that pre-CAD prototyping in the post since we built those first "duct-tape prototypes" a long, long time ago. So we don't have pictures readily accessible and it was harder to remember the exact process.
To start with, I think we modified another battery case and basically stuck batteries on the back to try out swapping.
I'll think about how to incorporate that step into the post and see if I can dig up some old photos.
I might have a pencil sketch or two, but I wouldn't be doing anything in CAD until I knew my idea was feasible and had some grasp of exactly how it might fit into a finished design.
What stages did you go through for the conceptualisation and prototyping of the idea? As someone in the hardware business, I know the circuitry involved is common and simple enough that you probably had little difficulty. But I'd still love to hear how you discovered what you needed where to source it.
Good effort anyway, we need more guides like this on the web!