That's what I did in mine but I used stainless steel folded 25mm all round
then predrilled the holes in the fold down and used normal plugs with
stainless screws,also spread fire cement on fold before fitting and it gives
a more secure seal.the folded area was slightly sloped out on all sides.It
is much harder to work with but as I have no liner in my chimney I felt it
was more secure.As you have liner mild steel would be more than adequate.

Alex