After conducting some analysis of the available internal space after removing the battery compartment, I am guessing this is going to be quite tight. I did a bit of modelling on Solidworks and I am reasonably confident that I will be able to fit all the physical components in (except maybe as many batteries as I might like).
I will seek to do a 3D scan of the internals to 100% verify the configuration, however at this stage I need to confirm exactly which components are required. I still have my rough plan from last month, and put together a fritzing schematic to identify parts to purchase.
Priority for the first prototype is to verify the Pi3 will be able to do what I want it to, verify button connectivity through ribbon cables and verify circuitry requirements for the joystick.
This will also generate a parts list for prototyping parts. This will come in handy during future builds.
I examined the PJRC website for example code and a schematic layout of the Teensy LC board being used as a USB keyboard. My intent will be to connect the Teensy LC as a USB device to the raspberry Pi, program accordingly and configure the RP to recognise button presses as key strokes. The schematic seemed to show a simple switch with 5V supply circuit back to the input ports of the LC board.
Joysticks and interfaceing with the Retro Pi operating system will be somewhat more difficult. I examined numerous websites that discussed using Analogue to Digital I/C chips to turn the analog signal into something usable by the Pi, or use of third party boards to interface. Noting the shortage of space within the enclosure, I want to avoid additonal boards if possible.
I found this album by MrWalkway which demonstrated that you can actually connect joysticks to the Teensy in a similar way to buttons.
Below is the first pass at the prototype layout.