2006 gmc sierra bed cut?

I have a 2006 Gmc regular cab sierra. I want to do a bed cut so I can house 3 JL Audio 13" woofers. I have been told that "it leaks horribly" I have been told that "if you use a compression seal and a normal seal" you will be OK. Lol I am curious what "seals" they speak of an if anyone has any expertise in this department. All help would be appreciated !

http://www.technologylk.com/_ED5381RL/CRL-Slider-to-Slider-Accordion-Boot-Seal-ED5381RL.htm?src=shop.com

Hint:  Cut the opening of the bed 1 inch larger than the opening of the hole cut in the cab.  Round the corners of the cutout.  Do not have any square corners, it is really hard to get the above boot to bend at a 90.

The reason for the 1 inch difference is to allow the bed / cabin to still flex.   Mount the box to the inside of the truck and on the outside, only support the box in the middle of the left to right span.  Make the seam of the boot wind up at the bottom of the opening.