[Christ]

For a 10", you can use 1/2" MDF and filler without sacrificing too much bass or creating too much resonance.

There is a mfr that makes very light subwoofers, also. I can't think of their name, but they're for street/track vehicles so you don't have to remove them without sac'ing a bunch of weight for a "standard" system. Alpine?

If you don't normally use CD's, you can make an audio interface that links from your audio-out from the MP3 player to a channel separator (don't mind the bad terminology... I can't think right now) and has a line-out to the sub. You can get apps for most newer MP3's that allow you to control channel functions, and some MP3's even have 5.1 already (and require a cable that is longer than a headphone jack). For instance, I'm fairly certain there's a 5.1 dock for the iPod that you can wire your speakers to. Check on it.

For your door speakers, you can take the ridges off the speaker frames, and you can also drill the "legs" on the speaker frames to remove weight without weakening them.

A ported sub box uses less material, thus weighs less.

Even through all this, you're dead in the water unless you're dead set on losing literally 2-3 lbs, more if you change the sub box.