The shrinking seal happen allot. I think what I would do is purchase a length of seal cut it to fit that space and secure the seam with black RTV. That is the easiest fix I can think of and yes leaving it open would allow water and bugs to get in there.
Jake my sidekick (yellow Lab) 10/04 - 05/20
2017 RAM 2500 CC 4X4 Cummins Diesel
2016 Flagstaff 26 FKWS
AF&AM & El Korah Shrine of Idaho