Use glass cloth not mat as the binder in mat reacts badly with Epoxy.
Although, to be honest if you put a new backing pad bonded to the hull, internally and drilled to the right size (just clearance) you could back fill the gap in the hull with epoxy filler, or if the new outer flange still has plenty of mating surface to the hull just bed the whole lot in Sikaflex. Why make extra work for yourself?