This is my first post in this forum so I hope I will be excused for errors and oversight.
Well in my opinion, for the 2 bits its worth, good quality bass is reproduced in rigid bins. Thus the higher the rigidity the better the bass reflex.
This being said, the metal on the doors is to some extent, pliable, and the thrust of the cone of the speakers working, would tend to produce audible vibrations, with the metal expanding like a balloon, thus reducing or even completely marring the listening experience.
There is an alternative though, if you care to think about it. And that is the central area, in the drop-down console just below the HU, near the bulkhead separating the cab and the engine compartment.
You will find ample space there to install your speakers. But you would need to do a little prep before the actual work.
Requirements:
1. 7" or 7.5"diameter PVC drain-pipe, the thicker the better with 2 pipe caps.
You can go in for broader diameters if the available space allows.(Make sure the length is 1/4" less than the with of the central console, so that it does not protrude and cause obstruction in free movement of feet.
2. Good strong wire-mesh with the length 2" less than the length of the PVC pipe, and width adjusted to what will go around the com[let inside of the pipe, with an overlap of 1/2"
3. Any kind of soundproofing sheet material, at least 1/2' in thickness. which will completely cover the inside of the tube.
3. Some strong nylon anchoring band, like the ones used to secure something on the pillion of a motorcycle, with hooks at either end.
4. Some normal adhesive and some sealant like silicon tube, used to seal cracks in between the window frames and and the house structure.
Now you are set for the actual construction:
1. Mark out the diameter of the speakers on the top side of the caps, and cut it out, making sure that the mounting screws are not in the cut out area, but that the speakers sit flush to the surface.
2. Drop the speakers into the hole created, and mark out the edges witha simple micro-tip marker.
3. Remove the speakers and spread a thin coat of sealant on the surface and allow it to harden. this will form a spongy airtight base for the speakers to be mounted upon, inhibiting air-flow and vibrations.
4. Now take the tube, and in the center of the length cut a 1&1/2" circular hole. This will act as the Bass Reflex Duct, allowing the punch to propagate in the front of the cab.
5. Now apply a small amount of the normal adhesive on the inside of the tube, in dabs which are not less than 1" and not more than 2" apart from each other.
6. Next roll the sound-proofing sheet breadth-wise and slipping it into the tube, open the rolls and pressing firmly with you hands, adhere it to the inside wall of the tube. making sure it covers the complete tube on the inside.
7. Now, slipping the wire mesh into the tube, open it out and interlock the edges with nose pliers or fingers, it that suits you, so that it does not collapse at a later time but keeps the sound-proofing sheet firmly pressed against the inside of the tube.Make sure the mesh is equi-distance from the edges.
8. All that is left to do is place both the caps on either end of the tube after applying ample adhesive and allowing he adhesive to harden under pressure, by placing the tube on its end on the floor so that one cap is on the floor and placing as much weight on the other cap (I used a simple tile and 8 bricks) for about 6 hours, or as recommended by the adhesive manufacturer.
9. Now thread the speaker wires through the duck and after connecting to the speakers (check the polarity) anchor you speakers to the holes in the caps, by drilling hole of a slightly smaller diameter than the self threading screws you will use, and tightening them till they are firm, making sure the screws do not slip. (Silicon will do the job of a firm contact)
Note:
1. Do not cut out the sound-proofing over the duct but simple make a cross slit in it. This will prevent dust and other things to drop in and harm the speakers.
2. Ensure that the speakers throw or push out, instead of pull in, on the beats, since the bin is too small to produce any kind of satisfying resonance on its own, and the complete cab will be used for this affect.
Mount the cross-over as piggy-back on the tube, and anchor the tube with the elastic band, making sure its as tight as possible.
Install the front grills of the speakers.
Advantages:
1. You now have a mini bass bin with double reflex, at mebbe a 10th of the price of an off-the shelf unit.
2. The integrity of your doors is intact.
3. Portability ensures that you can use this in your other car without much rewiring.
4. Since you would be using an active cross-over as quality is your prime concern, this is the first thing that goes. So in case of replacement you do not have to open the complete unit, since you mounted it on the outside.
Enjoy
Please let me know if you really did try this and if it was helpful.....
Much Obliged.
Cheers
Picasso.
Another 6.5 inches up front...? 
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