RE: What's the best way to make a hatch?
For a solid structure I use fiberglass
Cover the hatch area in film ( monocoat, oracover etc ) as a barrier. Over the top lay up a couple of layers of glass and once set you can gently remove. Cut out the shape of hatch and corresponding hole. Bingo one hatch that will conform to the shape of the surrounding structure. As for fixing I have found two small pieces of that rubbery fridge magnet glued to each surface works well ( bit of experimenting to get opposite poles ) I have seen those pop buttons they use on infants clothing used as well.
Regards Steve