would be the two-part epoxy putty for plastic that you can find at Home Depot etc. I have use the stuff on the bottom (outside) of my royalex boats, and it sticks very well - I even used it on the stems as a makeshift skid plate, and it stuck well there, and I did next to no surface prep. Only downside, the stuff I used was white, but who cares if the bottom of my boat looks like it was sitting under a roost tree. Be sure to get the variety that is for plastic repair (may also be called plumbers epoxy)
I'd try to work some into the crack, then a flat patch atop the crack. Its fairly cheap - $5 a stick or so, and very easy to work with and if it fails, no big deal - but I think it would work, especially if you roughened the surface and cleaned it well.
Custom Boat Graphics
Kayak & Canoe Outriggers
Kayak Motor Kit
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