The base is actually very simple to build. First, you take the hard wood (the 6mm tickness one) as the lower part of the base. Then, use some nails to fix the two bigger wood strips in the laterals of the base. You can use something else instead of nails, if you want, because nails are not that good (the base tend to bend a little and the lateral becomes a little loose). But I used nails, and if you use a lot (like I did) it won't bend.
        Then, pick up two of the other strips of wood and fix them in the top and the bottom of the base. Don't fix the other strip right now (the one that is between the Pad area and the buttons area) because if you're not that good at sawing wood (like me) the buttons will be a little (very little) bigger or smaller than planned, and if you fix that strip now, they may not fit correctly in their area.
        Below are some pictures of the base:

        Don't forget to drill some holes (1.5 cm diameter) in the remaining strip of wood so you can pass the cables thru it later. Also, you can make a cover using some remaining part of the hard wood. I've put two switches in the cover for select and start buttons, but I didn't connect them to the pad yet. The way you use to fix the cover is up to you. I used some headless nails so I can put the cover in its place and it won't move, but it's easy to remove it also.