Firstly I made a wooden former to hold the ducting. Some simple mathematics will give the overall diameter needed to achieve your desired finished size.
I decided on a 3.5 inch spacing between the two discs, so it worked out that my discs needed to be 18.8 inches diameter. With the 8.5 inch ducting I was using, this meant I would achieve my overall size of 34.5 inches. The 3.5 inch spacing was chosen because it ensured that the duct, once mounted, could not slip out too easily before it was hot-glued in place.
The two discs are easily cut out with a jig-saw from some thin flat MDF (Medium-density fibre-board).
MDF being the material of choice because it always comes with a good flat finish.
I then found a piece of Acrylic tube that was slightly larger than the inside of the duct. I then cut a section out, so that it fitted snugly inside with the ends butted together. Then using a hot glue gun I sealed the inside edge into the duct. This must be a good strong bond, as the initial forces when mounting the duct on the wooden former, will want to try to pull this joint apart.
This type of duct comes in a compressed form and will stretch to about twice its uncompressed length, so by applying a bit of stretch before assembly, you will find it easier to slip onto the wooden former. Too much stretch though and it will be loose, whilst not enough will mean it is impossible to mount.
I knew from the past that it's easy to stretch, but it's fairly difficult to compress back again without leaving it misshapen, so I took it slowly.
Once the duct was on the wooden former I carefully levelled and squared it up before applying a good solid bead of hot glue around the edge of the MDF's circumference (both pieces).
This needs to be a good joint as the fully finished toroid weighs 16 pounds (10Kg) once all the filler had been applied.
The sealing bead of hot glue mentioned above, will now need to be roughened with sandpaper to help the filler stick.
Once both sides have been glued and completely cooled, you can then cut away with a sharp knife the blob of temporary glue applied to the outside curve. The main task of holding the tube in place having now been taken by the two beads around the MDF's circumference.
Here you can see the unfinished 8.5" (left) and my current 6" alongside one another.
As well as the new 8.5 x 34.5 inch, I am also constructing a smaller 4 x 18 which may be needed underneath the main toroid, to help shield the top of the secondary.
Here is the last, and most tedious stage of the whole project.
The three inch wide foil I used can be difficult to apply without any creases, as you're applying a wide two-dimensional flat foil onto a curved three-dimensional surface. One inch wide would have been ideal, but would have taken for ever, so with hindsight I think two inch is a good size on larger toroids, and one inch on smaller ones.
Fortunately it will smooth out quite well with something like the back of a spoon being rubbed over it. You must though use a piece of paper between the foil surface and the spoon otherwise the foil will tear. I find the discarded backing paper of the foil itself is excellent for this.
You can still achieve good acceptable results with smoothing away creases, even if it looks awful when you first apply it, so don't get too disheartened with the initial creases.
You will not however get a nice smooth surface like that of a professional spun aluminium toroid, but the finished product will be smooth enough to avoid having lots of little break-outs that normally occur with just bare heating duct.