DIY smart watch...system?

So I had thought, aside from which board to put this in.

How possible/easy would it be to build your own smart watch, kinda?

So far I've found this little combo:
http://www.seeedstudio.com/depot/35-inch-Touch-LCD-for-Cubieboard-1-and-2-800x480-p-2234.html?gclid=CMbNzKmQhMUCFZKGaQodWj0A-Q

Which is a 3.5" display that comes with a cutie board, not sure how good the cutie board is.

But I'd imagine that could be powered by a standard cellphone battery bank right?

And then you'd need to make a case for the dispay, which could just be 3-D printed, and the maybe a housing for the board which would probably end up on your arm in some kind of holster.

Anyways just a crazy idea I had, would something like this be more functional than a smart watch? I'd imagine so, trick would be hooking it up to control your phone, and the size might be a bit of an issue.

I also saw some small OLED display around, looking into that a bit more.

Found this 2.4" OLED on ebay, it ships from china, but the seller seems to have an account in good standing.

I've got a Moto 360, and while the size seems a little daunting to wear at first, and that quickly passes, I can't imagine anything larger, especially after considering the housing and such. It would still make for a cool DIY project.