Flex Duct Toroid
I finally broke down and tried the Aluminum flex duct toroid thing. I used 4" flex duct from Ace Hardware. I gave about $8 for it (roughly £5 for my UK cousins). The center disk is two 12" pizza pans. They were $1.50 each from Fred's discount store. I used short pieces of 3/4" PVC pipe as spacers to hold the pans about 1" apart. Clear silicone adhesive holds the sandwich together. I wrapped the flex duct around the pizza pans and secured it with a bead of hot glue on both sides. The completed toroid is about 20" in diameter. Performance-wise, it doesn't work any better than my 4" x 20" plastic drainpipe toroid. There's not much more to say about flex duct toroids. They are simple, cheap, and serviceable. By the way, the last time I visited Home Depot (the mother of all DIY stores, you blokes would love it), I noticed they were selling 6" and 8" diameter Aluminum flex duct in 8 foot lengths at a very good price.