Tango Joe
is one Smokin' Farker
Simply Amazing!
Yes. You can buy a several foot tall section for a couple dollars. :thumb:
You can fabricate, but you can also buy all sorts of stove pipe adapters, elbows, reducers, etc. to make that transition.
I ordered all my stove pipe goodies from North Line Express.
You need to determine the correct stove pipe diameter to your oven size for starters. Then it's a matter of finding a suitable rectangular-to-round adapter/mount that you can anchor onto the chimney and cover/finish with mortar or whatever. I'm sure they're out there. Good luck, your whole project is looking so good!
I did a quick Google search, and found this:
http://www.fireplacemall.com/Chimney_Caps/Flue_Stretchers__Stainless/flue_stretchers__stainless.html
No worries, just putting some ideas out there.