That could be done and wouldn't hurt.
A seal could also be made, but it would take some work to do right.
I've been trying to come up with something simple(although it's been some time since I gave it any real thought).
A quick and easy way with a part that most everyone can find and buy cheap, just like the trunkmod itself.
Like a specific piece of weatherstripping that will mount without cutting or drilling on the tail.
Perhaps mounting on a frame fastened to the inner fenderwell sides/trashcan itself.
It would probably be simple to fasten weatherstripping to the passenger seat bottom, but the hump would be a challenge, if not impossible without drilling holes or glueing.
There ya go Russell, your turn to contribute to the trunk mod.
Get ta thinkin'!