The plastic was badly treed.
I reckoned the best course of action was to cut away as much of the plastic as possible so that's what I did. Later, I found that I had to remove a few turns from the top of the primary and raise the secondary up by 1", but that isn't shown in the picture below.
Primary with all extra plastic removed.
I don't have any objection to thin plastic formers such as PVC pipes. But the very thick HDPE that I used ended up being more of a hindrance than a help.
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