I'd probably get some ceramic paper or thin ceramic cloth and wrap the Cu tube with it, then wrap nichrome around both with many turns. I would then wrap this with many layers of ceramic paper or cloth and keep it all together with standard steel wire twisted upon itself. The nichrome should be in tight contact with the pipe, with the ceramic paper between them. Any other insulator should work, assuming it will survive contact with red hot nichrome and does not emit harmful vapors. Google alumina ceramic paper and go from there.