Can you clarify what you need, exactly? How big does the keyboard itself need to be?
like wrapping around? I'd suppose you could measure the distance around the cake, and the height. then roll your fondant out at least big enough to cover that. You could ''paint'' the keys on, using your color pastes. You could make sure the keys were squared by using a clean ruler lightly laid on the fondant, as long as you make all the rows just a little bit off-centered from each other.
i'm prolly not much help, but that's all i can really think of. 
sorry.
Just wanted to add on to Cinder's Idea with the ruler...you could also try to imprint the "keys" on with a small square cutter, like an imprint with diamonds but turned the other direction. Well I can see the idea in my head...guess I am not describing it right. Sorry
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