Have not built one into my camper, but designed some for ocean going ships. You need some gasket between your hatch cover and the coaming, I assume that’s clear. Then you need to apply sufficient pressure evenly across the whole gasket area to prevent water ingress. On the CLC kayaks, they use small toggles, but I doubt they will be able to sustain highway speed and vibration. Depending on the purpose of the hatch, I believe the design can become quite complex. How do you want to use it? Escape hatch? Additional ventilation? To be opened from the inside, outside, or both? So many questions 😉
Sounds good! I would fit a rim to the hatch cover in order to reinforce it, and to have a ‘pressure edge’ (non-english writer here, so I’m not sure about the exact term) that creates some interlocking with the seal. I doubt that an all-flat cover would be waterproof for long.