Quantity   Component name
1 × Moto X 2013 phone Other phones will work fine too, but mine is the same size as the register screen
1 × Royal ER 125 Cash Register Remove years of filth before using
1 × USB OTG cable 90 degree connector recommended
1 × Arduino Leonardo For converting the register keys to USB keyboard, and opening the cash drawer
4 × AA batteries But you have those already.
1 × Can o' Paint The spray kind, for plastic.
1 × 555 IC Prototyping Products / PCB Chemicals