Use a Dremel to cut a notch in the end of the top cap
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This should allow the clip to just fit inside.
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Turn a gash piece of wood to the diameter of the inside of the cap
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and cut a piece so that its just shorter than then inside length of the cap (to allow for the clip)
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Glue this piece onto the end of the tube not forgetting to put the clip on first.
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Glue to cap on and you're finished TA DA!