This is the most relevant entry I found related to your question:
“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.
We'll cover the following topics:
Basic shapes: Create circular or rectangular boards with precise dimensions
Rounded corners: Add rounded corners to rectangular boards for improved manufacturability
Custom shapes: Import external shapes for complex designs
Best practices: Guidelines for creating effective board outlines
Basic Shapes
Flux has built-in support for simple shapes (circular and rectangular). For complex shapes, please refer to the section on Advanced (custom) shapes.