Is this self-stick gasketing? Have you considered using the material that fits around the top half, and doubling up around the lower half? Close-up photos could help.
We reglued our old gasketing on our 1973 amerigo
...but sooner or later want to replace it, some auto parts stores carry gasketing, so do camping world type stores...but we haven't found quite what we want yet. Maybe it's not as crucial as we think to find the exact same type of gasketing...we'll have to face that when we get to it.
Hoping your project comes out great!