Blow thru questions

When you cut the hole from the cab to the bed what kind of material do you use to put between it so water and whatnot gets into the box or the cab? Plus if you were to weld a box from the cab to the bed wouldn't it flex and possibaly bend? I need to know what I need to do to get the best look and best way to get it done with out anything screwin up.

I used the rubber boot that you would use to connect a camper shell to the truck. Make sure you silicon it in so there is no chance of a leak. Also put the joint at the bottom to prevent any leaks. You can find it at some audio shops and RV/camper places that sell camper shells.

How big of a hole did you make for the blow thru?

it depends on how you want to do your blow through. There are 2 different way's of doing it. The conventional way is to cut out the whole back of the cab, but that leaves very little structeral integrity to the cab wall. Not recomended.
I do mine in a 6th order bandpass configuration, where only the port comes through the cab wall. Yes you can get the boot's at a RV or Camper shell place around 6-10 dollars a foot. Make sure you put the seam at the bottom, and silicone over the seam so it won't leak.