It is indeed most common to use a cast iron boot to achieve the installation you are working on, but as you mention, vertical height will prevent the use of a boot.
I am not aware of an adapter that is currently manufactured that would be short enough for your needs. It will most likely be necessary to have a fabricator manufacture a boot out of 304 or 316 stainless steel that can work for you.
Collin C. - NFI Master Hearth Professional
on December 7, 2011