Fireplace fresh-air intake w/good seal?

My masonry, wood-burning fireplace has a fresh air intake on the back wall; it's just a cast-iron plate that slides on rails across a round metal duct, which vents straight to the outside.
This makes it much easier to start a fire, without having to open a window, but it is quite drafty when it's closed - the plate has about a 1/4" gap behind it. Even with sealing fireplace doors, I get a draft in the room. I assumed the draft was coming from the (slotted!) damper, but after getting a top-sealing damper, I'm still seeing it.
Can anyone recommend an outside-air intake that seals well, or is there no such thing? An exhaustive Google search failed to come up with anything.
Jay Levitt |
Wellesley, MA | Hi!
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.