Thanks to Vonholdt and his blog I finally found out how to carve a 20-sided die which is something I wanted to try already a few years ago. The 5cm cube I started with wasn't perfect with few millimeters off but I just wanted to try if it is actually going to work. It was fairly easy with the instruction provided by Vonholdt but I would never figure it out without those instructions.
And here is the fully finished die. I am going to try another one at some point and will pay more attention to the starting cube shape and will also make the numbers bigger. I will probably make the numbers on the die as they appear on the MTG dice rather than D&D dice.