If it's like my Duster trunk lid, the inner skin is all one piece with no holes.
Set the trunk lid on some saw horses (so gravity is working in your favor). Put a stop collar on a drill bit and drill holes in the inner panel - on the ribs of course.
Get some urethane body sealer in a caulk tube from the auto body store and use a caulk gun to squirt a mess of it into the gap. It will take a long time to dry, but it will. In fact, any leftover stuff in the tube will be junk in a month or two.
Great stuff will expand and really mess up the outer sheetmetal. It doesn't take much force to move that sheetmetal skin.
Larry's idea sounds great too.