Joe : Yes you need to replace the gasket, there is a common braided style rope gasket that comes complete with a high temp silicone adhesive as part of the package
that you are looking for. Some packages say universal stove door gasket on them. What you want to do is to tease out the center strands that make this a round gasket
and flatten it out so that it looks like a Big flat shoestring. This will make a tight seal !
One of the most well known
Rocket Mass Heater builders tells an story about making a gasket material out of ''Wonder Bread'' that lasted for an entire heating season
and was replaced during cleaning in the spring !
Byron : using the paragraph above as an example of what else is possible, I still can not visualize your set up could you share a picture or a sketch ? Big AL