I have thought about this off and on, and for me the main problem with any mechanical link like a turnbuckle is that one would forget they were there when one opened the door.
One option which I haven't done anything about might be to make up some small brackets, 2 on the top corners of the door top, one on the top of the inner windscreen draught excluder, and one attached to the hood box somehow. Each bracket would carry one of those high-strength neodymium magnets, so that when the door is shut the magnets line up an clamp together
I need one of those Round Tuits, though...
