On the old Isobars I would use .008" stainless for the inner thrust tube and hold it together with 6-32 bolts at the seam (be sure to do it such that the helical exhaust blow over the seam, NOT into it!). I would use some thin aluminum duct work that I got at a local hardware store for the outer tube. I would cut a strip of the .008" stainless about .5" wide and just bend it by hand to make the Z-like thingy to offset the inner from the outer. One in the front and one in the back. Make it tight and friction is plenty to hold it in place. Its easy and never failed in any way. Be sure to use aluminum for the outer tube to save weight, it does not get very hot so Al is just fine.