I've got a thule rack, maybe not as heavy as a box, but still an effort to
lift. I stand facing the drivers side of the van, with my right hand on the
bar that lifts the top. Then, give the roof a big shove with my left arm,
and using the momentum, push the bar into place with my right hand. Its MUCH
harder for me trying to bench-press it into place.

cheers,
 Jeff