Our daughter did fall out years ago... head first onto a soft bag.  AND IT
HURT!!  Since then we devised a wood dowel rod with one of those elastic
cargo nets attached to it and put it across the gap.  THe cargo net can be
moved easily enough in an emergency.  The wood rod ataches at the mattress
across the width of the van using existing screw holes in bed replaced with
eye screws... attach the net across  the top of the roof lid using just the
rope portion of the net - no pole.

Madeline
15 Years of Westie Adventures
'90 Syncro Westie