What's this, some sort of USB-based docking expansion base for an x86 tablet (the only place you will have any luck whatsoever with an optical drive) like the Acer W500?

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Bingo! :approve:

The empty cubic would contain the MicroController necessary for converting the Duo KBD Switch Matrix Signals, the TrackBalls X,Y data and click output to USB.

Or... finding a new place to hide the batteries instead of rebuilding the original pack?

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Dang Betcha! }

Check the Duo Battery Thread link above for more details on this and two other configurations.