Foam blocks
I used soft foam blocks made from black urethane. They come on the door edges of new cars to keep the transporters from dinging them. I cut a rectangular slot in the block, and glued them to the skin over the flap hole. The rod rides up and down in the slot in the block real nice, and squeezes shut to block out any air. Any good quality dense foam scrap should do the trick, Works like a charm. Sorry, no pics till annual time!!
HTH,
Chris