OK I thought about this and I think I have an idea for you! What if you make a 'tunnel' of flexible netting attached on one side to the tractor, and on the other side to the exterior perimeter of the hatch door opening and to the exterior of the hatch door (so it won't get caught in the door when you close it). Sort of like the accordion connector between train cars that allows them to move yet stay connected. When it's open it will make a tunnel, and when it's closed it will make a sort of u-shape where it attaches to the van and door. I made this horrible drawing to demonstrate:
It will probably be much closer together than this but that's just to give an idea.
Janelle...
Looks like a really cool chicken force field!