This is an extension of another thread, but i wanted to show how you can make a simple block off plate which is quick, effective, and doesn't require drilling into masonry. Its not the prettiest, but it is out of the way anyway and will last. Ive been burning two seasons with the block off plate the installers put in. I finally went in myself and found how ineffective it really was, so after i did this im looking forward to an improvement in heat retention. I should add how it is done. Stuff with Roxul above the damper plate. Cut a 1/8 thick piece of steel and drill a hole for a long threaded rod. Two separate pieces of sheet metal, cut to the size of the damper area and made to fit around the liner. Slide the steel into place, and tighten the sheet metal up to the damper with wing nuts. Probably about a 30-45 minute job.