Bit of a long shot this one, but...

I'm looking at designing a spiral staircase (for a new-build). For various
reasons it needs to be possible to separate the floors the staircase links,
so I'm thinking of having a trapdoor that fits into the gap in the ceiling
formed by the staircase. The door would be hinged and counterweighted, on a
pulley controlled by a handle somewhere, tight fitting and fairly heavy (for
sound and thermal insulation), with a bolt to lock it. It wouldn't be moved
very often (once a week maybe), otherwise staying fixed open or fixed closed.

Obviously such a thing could be built from scratch, but does anything like
this already exist? Is there anyone who specialises in the building of
trapdoors or similar, who'd be able to give me advice on whether this is a
silly idea or not, or suppliers of components?

Thanks
Theo