I carry windsurfing stuff up there all the time.  If you're not 6ft and
pretty strong, you might need just a little help locking the top in place.
The initial lift is easy, it's the last foot that gets me.  Try and get
your box as far back as possible on the roof.

For lowering it, make sure the doors and windows are closed.  Air pressure
is your friend.  You'll figure out how much to open one window to allow an
easy drop.