I believe there is a thread addressing this very issue.
Specifically, look
here, or below.
That is the cleanest, lowest profile option I've seen. Since the sliding deck wasn't an option in the color I was after, the guy in that linked thread had the next best thing. I was rather happy that my finished piece seemed to come out just as good as his. It is a perfect fit between the wheel wells and to the tailgate, and simply lift it up and out to remove, if desired. I still need to come up with something to level the side areas out.
The Home Depot nearest me had two options in a 4'x4' plywood: 15/32" (which is what I got, and 23/32", which would also work great. The supports are 2x2s (1.5"x1.5"). 15/32" plywood sits slightly lower than the back seats, but any sort of sleeping pad will negate that effect and you'll never know it isn't perfectly flush.
Cut perfectly to fit at 41x43". Actually, I think I kept an additional ~1/4" over the 43" width for what I wanted. Measure for your own preference. I measured the supports to end just before the plastic trim piece at the tailgate so that they sit on the carpet and not the trim. Extending it would still work, though.
The carpet is The Home Depot's ebony one, or the only black carpet they carry, at least in stores. Note that it isn't a perfect match with the factory carpet, but still great. The ebony is a true black, whereas the factory has a bluish hue. I stapled the carpet and allowed about a 2.5" or 3" overlap on all sides underneath. I never did take a picture of the underside with the carpet on, which I should.
My sleeping pad and bag setup served me awesome in my 2002, where I slept in that for 677 nights. Only two in the new one so far. Many more to come. Just as comfortable in this one, but I don't like the power seats for this purpose--they're not fast to slide forward (letting precious warm air escape), and the seat backs don't go forward like a manual one. I knew this going in, at least the slow part, but it still works well enough for me.
My ExtremeAire compressor with a couple of kneeling pads level things out for my head area