If you wanted to build a box yourself, it's 2-3 days work if you're like me and have never done fiberglass before. I made a mold of the available cavity using a couple cans of spray foam. Kinda pricey, but at least you can get the thing out in one piece and do the fiberglassing outside so you don't have to huff resin for the next few months. I've attached a picture of what I did... basically mask the whole area off (ie. form a masking tape & paper shell in the box area), and fill it with foam. Then you can pull it out, start laying the glass on top, and trim/check fitment as you go. I used 1" MDF for the faceplate.
I'm running an Alpine Type S 12" w/ a 500W RMS amp. With a a couple mounting straps, and tying the faceplate & sub into the rear cargo area side cover panel, I get good thump with virtually no rattles until very high volume levels.
Alternatively, as for mounting subs in the doors, you should have enough depth to mount them with some basic MDF/plastic spacers. There is quite a lot of room in there to space the speakers out (and behind too). Not sure if the wiring is ideal for 150W RMS, but I think you'd be ok.
Personally I think a big sub in the back is a better choice than 4 smaller (you could probably fit 8s in the front, 2 of which I wouldn't consider small) ones only because there's less likelihood for rattles requiring bracing, but I know it's been done before with good results (and maybe some dynamatting).
