How to fit a damper in duct?
Say you have an 8 inch diameter duct.
Then the circumference is 25.13 inches (you can find the formula online).
Get a thin strip of paper 30 inches long.
Make two dots 12.56 inches apart on it (half the circumference).
Wrap the paper around the duct and tape it.
The dots are where you drill.
Sort of like this:  https://www.youtube.com/watch?v=1XsBwEmFO_w