Body Plugs for Trunk (71 Demon)
I'm liking the CB antenna theory.
Some things to consider anything you put there needs to be flat enough to not interfere with the trunk lip.
There are very flat flathead screws that you could put in the holes and a rubber washer on the back side. I agree that some RTV would be a good idea too.
In the short term you could put some clear packing tap over the holes to keep water out of your trunk.