In a nut shell, Automotive Weather Stripping.

I know that a lot of folks have this issue and if you are like me, I like to get my parts locally. Usually it seems I wind up getting the wrong parts sent to me when ordering online.
So this is a simple/cheap alternative to ordering a new taillight gasket (or seal).
It's the same material (although thicker) and you can get it at most auto parts stores and has the one sided sticky tape to stay in place.
Just cut to fit and or shape and put it on.