Get XTC foam baffles for the rear speakers... and line the inside of them with dynamat or a similar product... kinda make a small enclosure for them... and seal up the wire hole well.

For the front, ditch the plastic mounting piece... and fabricate a new one out of MDF... just make sure to make it extend enough from the door, so the magnet does not hit the glass when the window is down. Coat it with some polyurethane... probably a few coats to keep it from absorbing any water. Also, be sure to seal it to the door, and to the speaker... then dynamat (or again, a similar product, there is much cheaper alternitaves available) the entire door, inside the door panel... being sure to seal things up as good as you can.

Both of these methods create an "enclosure" for the speaker... I did this in my friends 2000 accord sedan, and you'd seriously swear he's running a small sub, which he's not.