Hi there fallenturtle!
Sadly I don't have inside pictures but let's see if I manage to explain it well enough.
I took off part of the cs64 keyboard structure (lots of dremel work) there, don't remember exactly but seeing the keyboard layout I guess it was the F keys + some of the top row and flipped it.
The hacked thing was put together screwed to a diy pcb with proper traces to register the keys, and pin headers to connect it to the core.
Btw. later on I noticed I could have gone with single key switches from an office calculator, I bet the plastic thing that holds the key in place is the same size or close enough to glue the C64 keys in place.
Search for replacement switches for calculators like this ones, they may work: