The biggest challenge was creating a frame that allowed the keys to be precisely spaced above the membrane. As I show in the video this was accomplished with a grid of Technic connectors and axles.
The second biggest challenge was finding appropriate printed tiles for all the symbols on a keyboard. Thankfully The LEGO Group has released all the main characters, numbers, and even a few special symbols over the years. I had to get creative with some of the keys though, which was actually quite fun. Still, there a few keys that could use some improvement.