hatch on our Scamp
is the left-rear window over the bed. The way you described yours, perhaps it's in the roof.
However, I did replace the seal on ours last fall
. I just used some stuff from Home Depot from their weather stripping section. It comes in a roll, is of a kind of white silicon feeling rubber and is in a cross section of two capital "D's" placed one on top of another, with adhesive on the flat part.
That's the best I can do in the artwork, but I'm sure it is intuitively obvious to all you smart people on this forum. I cut between the "D's" so I had two strips of one "D" that I used to replace the original seal. At each corner I cut a "vee" miter. It seems to have worked just fine. There is enough compression to give a good seal, and yet not so much that the hatch is difficult or impossible to close.
It's only been installed for a few months, so I don't know about longivity, and I hope I continue to not know for a long, long. long time.