[Cob] Making a firebrick chimney thimble

[Dulane]

I am wondering if anyone has tried using clay or fire-clay to create a 
sleeve or thimble where the chimney meets the roof. I intend to use EPDM 
pond liner, and I found these high-temp silicone flashing boots, but I'd 
still like to throw a little clay up there anyway to reduce temps.

I'm thinking of molding clay around a short piece of metal stove pipe, and 
then firing it in my wood stove. Then I would try and get rid of the short 
piece of stove pipe, and slide the fired clay over top of the stove pipe 
that goes through roof and into the silicone boot.

It may not be necessary to use the clay...as the silicone boot is supposed 
to be high-temp, but I'm wondering about fumes etc, and I'd rather use clay 
too.

I've read that they used to use terra cotta sleeves and thimbles...but i 
sure can't find them for sale on the internet.

Any ideas?