This image, looking north [original taken in the wet] is a mock-up of how the trapdoor would close off the open stair well. This will allow safe passage across it in the dark. The ladder now needs to be raised 50mm [ 2"] to bring the top tread flush with obs. floor level. Vibration-isolation clearance is required for the pier on the right.
Because the ladder is sloping [at 70°] it might be possible to extend the trapdoor to the left to reach between the handrails. I'd have to check the geometry to see if it is possible. 180 degree [flush] hinges would be required to allow the hatch to lie flat on the floor towards the south when opened. [i.e. Nearer the camera.]
I don't want a small step or drop to catch me unawares in the dark when the hatch is closed. Hatch closure should become routine during preparation for observing or imaging. A light/LED[s] in the lower room will ensure the hatch does not remain open after dark without the large hole in the floor being very obvious. A micro-switch to extinguish the light/LED[s] on hatch closure is not difficult to arrange as increased insurance against free-fall skydiving.
It might also be possible to extend the hatch slightly inside the pier cut-out. A simple safety bar could be arranged to pivot across the pier "kennel" opening when the observatory is in use. A suitable slot would allow the bar to rest on a screw when in the lowered/closed/horizontal safety bar position. A door on the pier cutout is also possible. The sloping surface would ensure it stayed closed without needing a latch or lock.
This is the original image with the stairwell open after rain. The large hole is an obvious danger in the dark. I left the jutting joists overlong in case I needed their support for some reason.
The floor hatch is likely to be quite weighty if made of larch boards. So a flush ring-pull handle [or knotted rope?] would be useful for the initial lift. A rope could be clipped into an open receptacle ring on the pier without compromising isolation. That would make it easy to control the hatch without stooping over the gaping hole in the floor.
There are presently gaps outboard of the ladder necessary for safe use of the handrails. The previous hole, between the top of the ladder and western wall, is now covered in floor boards.[Visible left beyond the top of the ladder.] There will probably still be a small open gap between the ladder handrails at floor level but it will be all but inaccessible to clumsy feet in the dark because of the jutting handrails themselves. My main worry is that I might be concentrating on the view in the eyepiece as I move across this danger area in the dark. Without some means of semi-automatic protection I am in increasingly likely to do a header if not protected from myself with increasing age.