Flu Pipe design

[UtahBeehiver]

There are a couple duel flues out on the forum somewhere, you are going to have to root around. FYI, Forno Bravo recommends a 6" for 32-36" so a 30" may get by with a smaller, say 5" or possibly a 4" depending on height.

[Taboga]

We are building our first brick pizza oven 30". Now at the flu design and building stage. We first had a 3 inch pipe, now i see on this forum we need 6 inches likely. So we will use 2 x 3 inch pipes together. Unless of course someone says nono thats not possible.
My question at the moment is, how do we best hold the flu pipes in place if not cemented, as that is not recommended i am reading?
i am thinking to make a sort of short chimney with rebar crossed at the base to hold the pipe in place.
what other ideas come up?
thanks!