My son wanted to make a guitar out of cardboard. He got a shoe box and cut it up. Then he realized that he had forgot to include one of the sides of the guitar. He knew how he wanted everything to work, but needed help to get this part right. I helped him create a pattern that would include all of the sides of the guitar and he cut it out.
The original:
After the cutting, we hot glued it together. He originally drew and cut out a neck for the guitar, but realized it would be too flimsy to actually work.
Instead, he asked for screws to hold the strings on. We tied on fishing line and used a match stick for the bridge. The guitar worked, but was really quite and not easily tuned.
I was impressed with how he knew exactly how he wanted to create it and how he adapted to mistakes along the way.