You might try cutting your own set out or
rubber. I did it.
It was amazingly easy.
You can cut them
slightly larger at the outside edge.
As the rubber squishes down in thickness it does two things.
It gets smaller at the metal to metal contact area but retains
it's resilience and wants to expand back to it's original size.
This action keeps it tight.
AND it stays original size at the outside edge (where you left it
larger) such that it resists being pulled into the plenum whereby
a leak might happen.
Another advantage is that
they can be used over and over and over.
The rubber resists sticking to the metal.
Here is a sight that sells flat rubber but I see Amazon now sells it too.
http://www.grainger.com/Grainger/rub...071&sst=subset