I believe this is the best video I have seen about repair/replace bunk end.
There is also discussion here
and a replacement door was purchased from FR and shipped inexpensively on this thread...
The black rubber seal can be gotten from FR bought by the foot.
Just to clarify... there are different styles of tents and how they fit on the ends of the bunk and on the bunk latches... depending on the year of assembly... understand this when watching the videos.
Many owners have put up gutters above the bunk doors to help keep water from intruding and there are dozens of threads on here about that... although if seals are properly cleaned and maintained I for one have not found it necessary to add a gutter.
I hope this info can start you on your way to fixing or replacing your bunk.