Replacing the light seals in cameras is a topic that has generated many pages of information.
I have little to add except that I have taken an observation on the use of 4ply knitting cotton as a seal and extended it. The use of cotton thread was supported by finding the original seals on a Practika IV - and, of course, I took a picture.
The advantage of cotton is that it is dead easy to run along a groove. It will tend to stay there on its own but I have used white PVC glue to help it along. It turns out that this is quite good and it can be removed more easily than some other glues.
Below are some examples of where I have used cotton.