"jim" wrote in message
...
we fitted stoves in place of our open fires- so not direct experience
per se BUT I do remember an inflatable chimney bung that would do what
you want - can;t remember the tradename tho - sorry!

From memory you inflated it with a bike pump and it was supposed to
seal it all up nicely - when you want to use the fire let it down and
out it came. Had a red ribbon to remind you it as there...

I'm not as mad as I thought then, I had the idea of sticking summat
inflatable up there.