To design this project I used the great KiCad StepUp workbench, specifically the design flow from FreeCAD to KiCad using the outline exporter.
First I began on the part design workbench, by drawing a sketch on the XY plane, like so
Drawing complicated outlines in KiCad is a chore, specially if it involves arcs that need to be tangent to each other, making it in a parametric sketch on FreeCAD is much easier and powerful. Notice the generous filleted arcs in between all transitions, when designing a FPC you really want to avoid any kind of stress concentration, as that could start a tear when bending or pulling on the board. The dimensions are thought out for fitting comfortably in the negative terminal of a AAA or AA battery, and the total bounding box area is just under an inch square, while being long enough to get out of recesses. The parameters defining those dimensions can be changed at will for different size battery holders or other requirements.
Once i was happy with my outline in FreeCAD I just switched to the KiCad StepUp workbench and pressed this button: "Push sketch to PCB edge"That opens a file dialog where i have to select an existing kicad_pcb file that has nothing in the Edge.Cuts layer (in this case my empty PCB file for my project). Once I reloaded the board file I got this on KiCad
With that outline in place I just had to place some dummy "test point" footprints and a shunt resistor. Then I used planes and free vias to fully route the board.