>What I'm looking for is simplicity, but secure space.

One clever idea that I saw might fit your situation, depends on how
you sleep in the Westy.  These folks were on a year long cruise and
did not sleep upstairs.

Remove the upper folding bed and mattress, fit sliding flat
drawer-like boxes in the space.  There is a lot of unused space
there.  Be sure to fix some sort of latch to keep them from sliding
forward with a sudden stop.  Simpler but not as convenient would be
to make a fence across the front edge and access the space only when
top is up.