I had a similar issue when my 'Cougar' slide was closed. There was no real gutter.. just a small one. I climbed up there and noticed that even with the slide closed there was still a small space between the wall strip and the rubber seal. I picked up a roll of closed cell self-stick weatherstripping (maybe two.. because my slide was almost 14 feet long) and stuck it on the wall (where the slide seal meets the wall) and that sealed it up tight!! If I remember correctly the closed cell weatherstripping was 3/4" or 1" wide and 1/4" thick. It worked well!!