Adapt 6" stovepipe to 8" chimney inlet- best way?

Ok you experts, heres a question you've likely discussed before.....could not find past discussions on this. Harman says it's best to run the same (6") diameter pipe all the way from stove to the chimney.My chimney was made with a 8" ID tile pipe. The space could be filled with cement though I'd like to remove the pipe easily for seasonal cleaning. Another way would be to do a 6 to 8" adapter with a very small length of 8" pipe though that is kinda not following the recommendation as far as going all the way with a 6" pipe. Ash would likely accumulate in the bottom of the adapter.....that does not seem like a big deal. Are there adapters that do this that allow the 6" pipe to go in all the way? Any comments, best practices? Thanks...

The best thing to do is reduce the thimble to 6" and run all 6" stovepipe, no reducers. A few inches of cement at each end should do it. Also, make sure nothing (thimble or stovepipe) actually enters the chimney, it should be flush with the interior wall.

Put a piece of 6" ID clay tile thimble in the existing 8". Use some lime-based mortar or portland cement mortar weakened with extra sand. You want to be able to get it apart if you need to.

I'm all set on doing this, Thanks for all the replies. Getting more tons this week. Lady said coal is hot, hot, hot, more Blaschak coming in, no problem getting, Price up from 245 to 255. Del extra.