Not sure if you should need anything thicker than 1" plywood........maybe even less. I haven't done it, but thought about it.
I envision lining the perimeter of the cavity of the stairwell with sheet metal screws at 1" below the surrounding floor height, or fasten the same number of "L" brackets. You would then rest the plywood board, cut to the inside dimensions, on the screws or brackets. To reinforce the actual board, attach a support arm (possibly a 2X2) to the underside of the board to wedge between the board and a step.
Does that seem like it would work to anyone else? I assume there is a similar arrangement for the entranceway of a lot of diesel pushers where the entrance steps are right in front of the co-pilot's seat.
I would keep the screws or "L" brackets short so as to not snag your ankles on them when using the entrance steps normally.
Ken and Terry
2010 Georgetown 330TS being pushed by our
2017 GMC Terrain toad (4 wheels down)