I like to use the stirrers in Dunkin Donuts. My tube flys are used in Salt mostly and are about 1/2 inch long at most. The Dunkin Donuts tubes have two holes and take a 15 or 25 # test line nicely. I use a plastic coated large paper clip in my vise to hold the "tube". I guess your fiberglass tubes will work well since they should be really stiff. The question is the inside diameter of the tube and what can you hang it on for the your vice so you can tie it.
dc

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Many men go fishing all of their lives
without knowing that it is not fish they
are after.

Henry David Thoreau