Final product! After a long time just drilling and sanding it down, it was done! I assmebled it and placed my laptop on it to see if fits.
Overall, I really enjoyed this project even though I made many mistakes along the way. My final product isn't perfect though. One final mistake I made was that I made the slots a bit too big to account for the wood. I was really afraid the wood would not bend like cardboard so I won't be able to force it in the slots. I made all holes a few millimeters bigger but in the end, my slots were all 0.01 too big (just a paper width too big) so it was a bit shaky. However, if I just put some tape in the slots or stuffed one pice of paper in it, it won't shake anymore.
In the end, I decided against to glue everything together to make it not shaky because I wanted my stand to be still easy to assemble. At home, I just put a thick tape (like masking tape) and paper in the slots for a temporary solution. I guess the phrase "measure twice, cut once" is true. But, regardless, it is still useable and I am proud of my first laptop stand.