ZC fireplace adaptor plate question

I have a Napoleon NZ26 with a 6" outlet. I bought the DuraPlus adaptor plate since I'm going to run triple wall right out of the top of the fireplace. The actual O.D. of the adaptor plate is 5-7/8" but the I.D. of the fireplace is 6-1/2". Can I just put a piece of rope gasket in to take up the space? Or do I need some reducer? If I put in a reducer I will not be able to bolt the adaptor plate to the top of the fireplace box.

Thanks Begreen. As it turns out, I have the base plate, not the anchor plate. I spoke with Napoleon customer service who said I need a starter collar. It did not come with the fireplace, and I have seen some for sale on line, but no one has a picture available on line to show what exactly it looks like. I am wondering if a short piece of single walled pipe would be sufficent to take up the space.