I have a similar setup with an old Rhode Gear rack.  Seeing the
possibility of the rack coming unglued for any of a number of reasons, I
ran a length of nylon webbing (optic yellow) all along the top and down
to the front bumper, leaving it a tiny bit loose.  If the rack came
loose, I would see that bright yellow rope cinch down in a hurry!

You can almost guarantee that the rack won't come loose if you remember
to tighten the webbing after ten miles and again after a hundred.

Karl Wolz

Also, those racks have a tendency to unclip the bottom straps if you
hit a bump (the rack shifts down, and the strap loses tension and the
clip slips down off the bottom of the door).  I put a bungie cord on
the two bottom straps as well to keep them under tension.