roof vent/ fan

I am getting ready to make the first cut in my roof
to install my fan.
Having to deal with a leaking roof in my house making this first cut is hard for me.
Looking at the radius of the roof I will need to add spacers at the front and rear of the fan flange.
Does anyone have any ideas on what is a good product?
Steve G

Having to deal with a leaking roof in my house making this first cut is hard for me.
Looking at the radius of the roof I will need to add spacers at the front and rear of the fan flange.
Does anyone have any ideas on what is a good product?

Steve G