When you get the seal you will look at it, offer it up and think, how the hell does that work.....
Hopefully the diagram below will explain.
The seal stands up on it's end and just hooks behind the lip on a spot welded plate on the upper side panels. It doesn't stay in place that well but should hang on long enough for you to flip the panel over and get it in place. Once there add the nuts and start tightening it down but go along the panel from the inside pushing the seal outwards and up against the side panel. As you tighten the nuts it will stay in place and compress to form a seal.
You are correct in saying the seal is just a roll. I guess you could cut the seal, mitre the corner and glue it back together but all I did was push the seal into the corner forming a tight curve around the outside of the corner fixing hole. It will make sense once you get all the bits together.
You may also notice on my photo below Land Rover originally used lots of black glue. I didn't replace this just fitted the seals dry.
Craig.