I had the same sort of issues with my aero cap sealed to the cab. I'm guessing that since yer extension is moving/shifting, any seal you have attached to the car side will eventually break free. I tried securing lots of different things to the cab side with little success. I finally went with gap seal on the aero cap side. Keeping it tight to the cab was key to keeping water from leaking past the seal. I used some bungee cord to keep the tension on my gap cover...
http://ecomodder.com/forum/showthrea...tml#post314394