In-Game Only
This video shows you how you can create circle in-game without any third party tools. There are some very specific steps involved here and you'll find there are certain things that just aren't possible and the alignment is not 100%.
Using Satisfactory Calculator
The Satisfactory Calculator Interactive Map (SCIM) is my preferred way of creating rectangles, circles and roads. By using SCIM you will have 100% foundation alignment and it allows for shapes of any size.