Designing Custom PCB shapes in Fusion 360
The outline of this PCB was created in Fusion 360 using the spline tool. You can create unique shapes with curves and arches using the sketch tools. Use the sketch dimension tool to define exact measures. Then, use the DXF export feature to save out the sketch. Eagle CAD supports importing DXF sketches. Using similar canvas coordinates across both programs, you can get exact placement when importing the DXF.
Designing in Eagle CAD
I started with downloading the Circuit Playground Express board file from github. Using Autodesk Eagle software, you can open the design and inspect components, traces and the pin layout. From there I referenced the size and positions of the pads. I then created a simple layout with the exact same pin layout.
If you're new to eagle, I suggest watching a few tutorials on youtube to get familiar with the user interface and features.
CPX EAGLE Library
Once I laid out the traces on the board, I wanted to create a reusable object of the CPX pin layout so that I could easily create new shields and adapters. As a single object, it's much easier to place it into a new design instead of recreating it each time. The library can be download and added to EAGLE by opening it and adding it to a board design. It features the "device, package and symbol" configuration so it'll properly connect to nets in the schematic layout.
Feel free to download the library and use it in your projects!