More DIY
The screw sounds like a good idea. Unless its on the keel that would be exposed to the ground (dragging on the beach, loading etc.)
Another option would be to get a plastic welding kit (think they have reasonable prices on them at Harbor freight) Then you could practice on a scrap, or somewhere else on the kayak to get the temperature and technique right.
Otherwise, as always, oex is a great option.
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