no need to build a device just use a USB stick MP3 player such as
http://www.1topstore.com/en-gbp/2gb-lcd-mp3-player-usb-flash-drive-built-in-fm-radio-p8840.html and just change the firmware..

or secondly if your just trying to prevent someone from just using your pc to hack coins by requiring a USB stick to confirm payments..then just use a plain usb stick. make it so the new client wont transmit payment unless a USB stick is inserted containing the wallet or even a special file which the client references. then just dont plug it in untill u need to send payments.

or thirdly why even need a extra usb stick hanging off your keyring.. phones have bluetooth and wifi these days so transmitting a transaction does not require a USB stick insertion into a PC and if u were thinking of using it to plug into merchants POS terminal at stores. it saves risks of some savvy merchant adding a trojan to their POS terminal to clone wallets plugged in.