Gap filler recommendation

I’m installing a metal roof over 2 in of polyiso rigid foam insulation over an existing asphalt shingle roof.
I’m looking for a recommendation to fill the following gap: between the metal boot that surrounds a triple wall pipe that connects to my wood burning fireplace
flue.

The product needs to adhere to the polyiso foam board as well as the metal boot and last a long time.

Is there another product as cost-effective & long-life as roof cement?

Thanks for any help.

Butyl if it won’t see daylight. Polyurethane or a terpolymer if it will. Roof cement gets brittle when cold, metal moves when cold… don’t use it. Silicone cures to itself and in a couple years divorces from the substrate.