I gots a ideas cut 12"-18" hole in cab and bed to match then use either flexible duct for heating and air or they make boots u can get at the rv place, the blow up kind ,but whatever u use I would only use the size hole I needed for the speaker not just open both sides up !!Also bass has no direction meaning if u put a sub in a room its hard to tell if its in the front or back or pointed left or right u might be able to cut in the speakers into the bed like u want and not even connect them to the cab ,a pair of 15s with a few hundred watts will sound the same in ur truck or in the car next to u anyway , .U could use the boot to seal out any moisture from the speakers and if I am not mistaken most subs are plastic cones and rubber surrounds anyway u might not even need the boot !!!!Just cut em into the bed and see if u even need to do more,next are u going to use a sealed or free air set up or ported they all have a little diff characteristics or are u lookin for the cool factor of opening the bed to the cab?????also do u know theres a big difference on how they will sound(hit) from the shape of the box ?????a pair of 18s will need almost the entire bed to sound proper a pair of 15s will use a little under half and for what ur doin I wouldnt go much smaller or it will sound like 2 10s under the seat!!!U can slap anything together and get sound but theres a big diff in sound and sound that sounds good!!