Here’s what the Flux documentation says about creating and modifying PCB board outlines:
"In this tutorial, we'll explore the different ways to create and modify board outlines in Flux, allowing you to customize your PCB shape and size to fit your project requirements.
The board outline defines the physical boundaries of your PCB and is one of the first design decisions you'll make. Flux provides several options for creating board outlines, from simple shapes to complex custom designs.
Flux has built-in support for simple shapes (circular and rectangular). For complex shapes, please refer to the section on
Advanced (custom) shapes.