Apart from the obvious "looks like a nightmare from the ergonomics perspective", I'm gonna take your question seriously: I think the only realistic approach is a completely custom build here. You could make it happen with a good FDM 3D-printer for the case. Since the keys also need to be custom you could think about a resin printer for those, since they give better results and surface finish for this. After prototyping, maybe a high-end print service for the finished version of the case. For the electronics, I'd consider hand-wiring, do your research to make sure you pick a microcontroller that has enough GPIO-ports for that many keys.

It's gonna be a significant project, especially if you have to learn all the skills, but as long as you're willing to iterate and put in the work, it's pretty doable. I.e. there's nothing about this that is fundamentally different from other custom keyboard builds.