A "yacht hatch" type design might be suitable:

http://jp.inmagine.com/pdre019/pdre019074-photo

These use an overhead sliding hatch and and a smaller lift-out
vertical section.

The advantage is that the hatchway stands a foot or two above the
floor (deck) it opens onto, so less danger than of a gaping hole - and
it provides containment for the top of the handrails - without
interfering with the operation of the hatch.