Going to make my own register plate for a stove. Thinking of using Superlux board or similer instead of sheet steel. With regards to fitting it, do you fit angle brackets to the inside of the fireplace and screw the register plate to those?

We did that

Did you just have a single piece board with a hole for the flue, or a two piece register plate (i.e. one half each side of the flue with the flue hole being cut in two)?