What filler to use between rooftop and liner?

[austinrick]

I am getting close to cutting the 5"ø hole in the coachroof to accomodatae the 3" flue pipe and deck fitting of the Dickinson solid fuel heater I'm installing in my Bayfield.

Dickinson is rather vague about how to fill the resulting gap around the perimeter of that hole, prior to screwing the deck filling in place. Whatever I use should be able to flex a bit when a foot lands nearby, be drillable, but still not compress too much when I run some screws up through the ceiling to anchor the deck fitting, while also not being too susceptible to heat.

My question to you is, what material would you recommend to fill that gap? I'm guessing it will be about a 1" or less gap around the perimeter of the 5" hole.

I have not had a chance to use West Systems products, but see a 404 High Density filler that seems like a candidate.

I welcome your thoughts.