Thanks S-F
I was starting to see potential problems with just boxing in the combustion chamber. For example, a fan would be needed to keep the fresh air flowing in, but it would have to compensate for different burner flame sizes (my boiler's burner adjusts the flame between 11-23kW) - too much air with a small flame could be wasteful and/or dangerous.
Replacing the innards with those from a closed chamber model probably wouldn't be cheaper than buying a new boiler.
So a pipe that ends right under the boiler it will be. But the damper which closes only when the flame is off should be OK?