If you do it with a teensy or pro micro, then you could make it a fully functional numpad as well as a calculator. You already have caps and switches, so you need a case (a plate at least), diodes, teensy or pro micro, and an LCD screen. There's a ton of options for LCD screens out there. There's also already libraries to come the scene so the more difficult closing is done for you. I would also recommend getting a dip switch to switch between calculator mode and regular numpad mode of you want access to both modes.