I have made custom tubes by cutting down larger ones and by inserting strips to make larger ones. Cutting down a larger tube is easier on smaller diameter tubes. If you pick the right size, you can use the strip you cut out as the backing material for the seam and it will already be conformed to the tube shape. You can still use centering rings and cones by cutting a notch to fit the backing strip. This is an easy step. This method should be slightly lighter than building up a tube, assuming gobs of glue aren't applied.

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I love sanding.