I have been teaching myself how to 3d design in fusion 360 and while it has not been a quick or clean process, I have found it very fun. If you are really passionate about it, I recommend taking a few measurements of a PCB you want to use and designing the case around that. You can also look at keyboards that you find interesting or relate to your design for inspiration. Also be sure, when designing the case, to write down every single measurement. Wall widths, filet, typing angle, and other stuff like that will be necessary for general consistency when working on a project that may take months. Those are just the main things I would have liked to be told when I started my design process.