"The Great Stuf could be trusted to push out the panels and maybe loosen the fasteners."Even when it says on the can "will not distort window frames"? 
It was kind of a generic suggestion - a layer of 1" insulating foam bonded perhaps, or something else to increase the section thickness.
The woofers will Adire Tumults - they're currently in 17" H-Frames, but I want to see if I can "kick it up a notch".