Advice needed to accurately cut foam insulation for pool bottom.

[springer01]

I am hoping to start my pool bottom foam insulation tomorrow, but I am having a hard time figuring out the best way to accurately cut the foam for my 21ft round saltwater LX agp . I tried to search, but couldn't find any answers. It looks like from pictures people are cutting it with the walls already up, but that seems like it would be super difficult to get right. Thought about using the bottom rail for a template for a rough cut, then taping seams and making any minor fit cuts after the wall goes up. Any advice would be greatly appreciated!

Thanks
Tanner

 

[TomGallopavo]

Put them together and draw a circle with a string and sharpie. Reassemble in pool.