Last week's trip to REI, spotted these two
aluminum tent pole repair-sleeves (13.2mm and 16mm diameters). Don't have in front of me to measure, but they're around 6-7 inches long. At 75 cents each, bought one of each, thinking could cut to length and come up with suitable end-caps for some as-yet-determined kit use.
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"Things that have never happened before happen all the time." — Scott Sagan, The Limits of Safety