The message
from "Mark D." contains these words:


The Fireplace Plug is a clean alternative - no need to get covered
with soot when installing the plug as it is quickly installed below
the damper as opposed to up in the chimney. Available for about $50
from www.batticdoor.com

Perhaps less appropriate for many UK fireplaces which don't have any
damper like American ones to -- I can see it simply going up into the
smokeshelf area and doing nothing useful there.