How much time do you have. Edible image would be cheaper, but you can always get a custom stencil made that you can use over again. Depending on the company and detail they start a $20 (just letters), with lettering sizes as small as 1/4". Either of the whole keyboard (custom design where you provide the drawing) or individual letters, then your royal icing could be much more precise.
Yes, the laptop wasn't meant to be so realistic. The first time I did the laptop cake the plan was to take some letter keys make a mold out of them, then get a custom stencil. When it was all said and done, the cost was too much for the customer. So this less precise method was arrived at.