How’s your carpentry skills?
If the answer is “RS”, do you know anyone who has some experience with plywood?
My suggestion is to make a mirror image of the cavity formed by the internal steps — a stepped box which drops into the steps and creates extra flooring over the cavity.
Either a pair of rope handles or two low profile handles on the sides to pick it up.
No slot grab-hole: too dangerous.
Then you can pick it up, turn it upside down and place it outside.
You can work out for yourself where, how and what type of handles suit you.
SIMPLES!
ps. If you want, you COULD hinge the top/floor panel (must also be latched!) and use the box for storage of LIGHT items which can survive being inverted! ie. NOT lamps or fragile stuff.
pps. It might be a bit complex (since when has that stopped you?) but you might be able to work out a hinging arrangement to permanently mount it to the bottom step!