Is that actually a circle though? At 90 degrees if you project a circle onto a cylinder it approximates an ellipse (if you unroll the cylinder's surface). Other angles make egg shapes. You can probably jam it in there because it's just ducting, but for materials more rigid than dirty old tin this method won't work.

This is a neat website. Give it some parameters and it'll spit out a printable PDF template you can use to cut your pipe.

https://cq.cx/tubejoin.pl