Thanks a lot for the suggestions. As it turns out, I created an unmitigated disaster! The epoxy was too hard for the fishing line, or even a wire. Too hard for a knife as well. I then tried a Dremel which seemed to work fine at cutting through the epoxy in the gap. But there was too much leakage as a result of epoxy that wasn't thick enough when I built the hatch stops. No amount of prying (I really liked the idea of a bent putty knife) would do it. The amount of heat I would have needed would have delaminated the fibreglass on the hatch cover.

So with a grimace on my face and a stream of choice words, I slowly cut the whole hatch out with a jigsaw. The true extent of my mistake was obvious when I got the hatch off - there was so much leakage that I can't yet get the masking film off the underside of the cover. Clearly I was over my head trying to use magnets - now it'll take me days to clean up.

If it's true that you need to make mistakes in order to really learn, then I just earned a graduate degree.