I bought an chimney balloon that fixed a similar issue in a zero
clearance fireplace that had a leaky damper. If you know the rough size
of the area above your damper or if you can measure it with a folding
carpenters ruler or tape measure you get a balloon that is apropriate
for the size of your chimney. Insert the balloon through the damper
opening and inflate it through the valve tube. You can find them at
http://www.chimneyballoon.us or I see northlineexpress.com and amazon
carries them as well.

I recommend the clear balloon version over the black balloon version
since you can buy a clear one of an apropriate size for your damper and
it installs into the chimney so you dont have to see it.