Previous posts suggest screwing the plate to the chimney.
Errr how? The simplest I can think of is bending over an inch and using
rawl bolts ie all metal construction, a bit OTT but fireproof. The inch
forms a tab parallel with the chimney wall in order to screw through.

Presumably prior to that you connect up the liner to stove pipe and
afterwards you seal up the plate with fire proof sealent and then pour
vermiculite down the chimney. Top bit I'm okay with.

Would aluminium register plate be okay or is it likely to be too hot even
for that?

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Mike W