The damper hardware by the firebox is too intrusive to fit any plug below the damper (image 28-2). If you go below the damper hardware the hole is very long and wide so nothing will fit there either.
The only option with this application is to install a Chimella above the damper. The Chimella has a 2 foot long handle, but that may not be enough to reach the flue tile. So I would recommend you use a Large Chimella and put it above the damper (image 28-1). Just make sure the location you put it is no bigger than 19″x25″.