Since this is just a early prototype, I decided to work without any PCB.
I decided to employ ADNS-9800 as a mouse sensor and Teensy 2.0 as the main controller board.
Wiring is done with some old IDE cable (yeah soldering is not my talent ;-).
ADNS-9800 switched to 5 V.
Cherry MX keys (I've chosen Blue ones for its soft clicks) are mounted in the main body.
Keys are wired in 3 rows by 12 keys in each row. This occupies more pins but reduces the ghosting problem: a finger cannot press multiple keys of one grid column.
Thumb keys are wired to the middle row on their own columns.