I just built the base with the keyboard slides, and I'd like to suggest one alternate way of attaching the slides:
First, you build the box. Then, attach these convenient brackets provided with the keyboard slide kit...

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...to both sides so that they end up even. Remember to extend the slides as much as you can. This way you don't have to measure and screw things up when your measurements don't come out right.

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Now, with anything convenient and hard, bang on the ends of the screws attaching the brackets to the slide to make an impression in the wood. Here, I use a wrench inappropriately.

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And drill out a bit using the large bit so that the screw heads no longer get in the way of the brackets being flush against the wood.

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Good going, dude. To permanently attach the brackets, do NOT use the screws that came with the kit. They are small and weak and will rip out under pressure. Use the big wood screws that you used to put together the box.

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You made Mr. 19th Century happy.

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