I'd get some lumber and build a rectangular thing that would fit into
the empty space, put padding on the top, and cover with heavy cloth so
it doesn't scratch anything. It would be like a little extension of
your box springs and mattress. Doesn't matter if it's real comfortable
unless you're 2 meters tall. Then you could use king-size sheets and
mattress pads.

The only catch is, what would this rectangular thing rest on ? Is
there any support underneath ? May just need to add another rail.