Benen Huntley

It looks beautiful and fits in great with the earth dome. I hope you're proud!

Glenn is correct. It is discussed on Peter's batch rocket site batchrocket.eu on his most recent build. To simplify the build they did a square riser with only chamfered rear corners up to at least the height of the port.
He reported that performance was at least equal with an entirely octagon riser I believe.
I have made a 5" system for a friend this way recently.

I have been a part of two wood heater saunas. The second was a small sauna with a 4.5" batch rocket. The thing cranks to 130c with that heater. We find that between 65c and 80c to be a good balance so it is more than enough power.
Yours is a lot larger but I would think even a 5" would do the job.
Adding mass will cause it to take a very long time to heat so I would advise just using a barrel with some rocks on top. That way you can still use water on the heater but get instant heat when you light the fire.

I'll give a strong recommendation for a batch rocket. I'm biased, having never built a j tube. While I agree that the j tube is simpler to build, I can't say I found the batch rocket particularly difficult to construct. I was offered plenty of excellent and prompt advice here at permies and took my time and I'm very pleased with both of mine.

It's alive! Still drying the bell but burning smokeless and the house rose from 10c to 20c quite quickly. The other side where the bell is got up to 30c quickly and I can't touch the top of the bench. Water steams off immediately.
Need some shielding behind the barrel urgently as the wall is getting very hot!
Couldn't be more pleased!

Picked up the glass for the door today. All that's left to do is the cowl and dektite on the roof, get the glass in the door and drop the bell over the riser.
Was hoping to have it firing tonight but I can't lift the bell alone.

Nearly there now! Monday the flue will go in and tomorrow some paint.

Perfect. I just had concerns regarding proximity of the flue to the barrel and the length of the bell. Thanks for easing my mind.

Is that right? Damn. I had 1" in there and took it out.

Here are the sketch up files.
Basically it's passing through a wall in the dining room. The bulk of the heater is in a hallway but there is plenty of venting space for air to get to the living area. Will probably use heat transfer ducts in the ceiling to move the heat to other areas of the house.

https://drive.google.com/open?id=1GXikJzH31MpAYpSUIZED61-loy5w5yi1

https://drive.google.com/open?id=1UVarMtfpQjINe1Fvs5H_JZ1HYIdO10qt

If I use 1 inch super wool then the diameter is much too large in the riser.
Haha yes, I'll use sketch up to draw the bell in place to show what I mean.