Bob,
I don't see why the outer tube wall thickness need be very great. 1 mm would do.
Here are a few figures that would seem to be somewhere near.
Centre rod is 1/2" diameter. gives 9.426 sq ins for 6" long.
Tube has 3/4" bore. Gives 14.14 sq ins for 6" long. (internal surface)
So the maximum surface area is 14.14 sq ins.
The easiest way to make this thing is to get a bit of 3/4" bore stainless pipe. Everything will be nice and even. No messy welding to cause distortion.
Just a thought.
Manta
p.s. The sun is shining, the weathers fine. I'm off to make some biodiesel. You guys remember that stuff ? Very popular a few years ago.
