Have you tried putting some weather stripping around the gaps ? the stuff I'm referring to is the same foam material used when sealing a truck canopy t the bed of the truck. It comes in rolls so you can cut it to size.
SO WE WRAPPED 'E UP IN A MAINSAIL TIGHT
WITH TWICE TEN TURNS OF THE HAWSER'S BIGHT
HEAVED THEM OVER AND OUT OF SIGHT
WITH A YO HO HO AND A FARE-THEE-WELL
A SULLEN PLUNGE IN A SULLEN SWELL
TEN FATHOMS DEEP ON THE ROAD TO HELL
BOTTLE OF RUM!