Well, after much thinking and a mountain of "honey-do's" I finally got the time (and some decent weather) to build my sleep platform.
Special thanks to Jnelson from this board for giving me the opportunity to look at his and get some ideas.
Since I'm not much of a carpenter, I decided to make a very, very simple platform using a couple of sheets of 15/32" sanded plywood (sanded on both sides, so it's pretty nice.) $32/sheet at the local Home Depot minus 10% military discount, it was the only out-of-pocket expense I had, as everything else (2x4s, 2x2's and 1 1/4" wood screws) were things I had lying around the garage.
For the benefit of those who might be contemplating something similar, I'll show you the process.
Started off by getting the plywood cut at the store. Most of the big box stores will give you 1 or 2 cuts free. I had each piece cut once, at the 6' point on the long side so it would fit into my DD Mazda pickup (also I knew I wouldn't need a peice longer than 6'.) This resulted in my two 4 x 8 plywood sheets becoming two 4 x 6 sheets and two 4 x 2 sheets.
Once I got them home, I measured for the two "bulkheads" on the sides. This will carry most of the weight.
Before you get started, you need to remove the bottom cushions on the back seat. This is easily done with a 14mm socket and the seat cushions lift straight out. Then drop the seat backs down.
The thing that makes this a bit of a challenge is that dropping the seats does not make a flat load deck. Instead, the seat backs actually slope upwards.
I cut a 6' long peice to 10 3/4" high. I then measured where the "up slope" started and how high it went. This gave me a good angle to cut, and then I test fitted it. It needed a little more "angle" so I cut a bit more (I've learned the hard way, cut less rather than more.) Final result: It fits and is level on the top:
Once I had one, I used it as a template to cut the other one. Having two, I then used 2 2x4's to connect them together.
Note that this is flipped upside down on the garage floor. This way the cross-peice is flush with the top of the side pieces. Once two of them were on, I put them back in the truck for a test fitting (I did this a LOT. Again, I learned this lesson the hard way!)
After this, I measured for the bottom crosspieces, and screwed them in as well. After test fitting that, I put in the rest of the crosspieces. Here is what the "frame" finally ended looking like:
Not sure of the weight but it's pretty easy for me to pick up and move around. I'd say not more than 50lbs max. I ultimately went with 4 crosspieces on the top, three 2 x 4's and one 2 x 2, and 3 across the bottom, two of them 2 x 4s and one of them a 2 x 2. Probably a little "overbuilt" but I want it to be strong. I had originally intended to put a plywood panel across the bottom instead of the 2 x 4, but quickly realized the 2 x 4s would be much stronger.
For the top, bed-area, I took the other 4 x 6 plywood sheet and cut it down to a little less than 4' long, and combined it with one of the 4 x 2 sheets that was created when the original sheets were cut. The smaller sheet will go towards the front, while the larger sheet goes to the back.
You can see from this picture that the smaller, 4 x 2 sheet, warped quite a bit after it was cut. This may be corrected by putting it on a long piano hinge so it can swing upward.
The other thing that has to happen is I need to get some indoor/outdoor carpeting and cover the platform. Eventually, the big sheet may be screwed to the frame, or I may just put a hook on it and use a turn bolt to connect it to the floor, not sure yet. Being as how the "bed" is only 4' wide, it easily slides in and out of the truck.
The forward end of the platform needs a slight "lift" to make it level, so I cut these legs from some 2 x 2 I had.
However, I will not permanently attach these legs because it will make it more difficult to slide the platform in and out. Instead, my thought is that while in "travel mode" the legs will remain detached, and then when I get to my campsite I can prop up the front of the platform on the legs. It will only need to be like that when I'm sleeping. I may use wing nuts to attach it or I may just leave them propped up. Since I'll only be using the legs when I'm "static", I'm not sure they need to be attached with a bolt.
My platform will accomodate my super-dooper high speed, high tech 'drawer' system:
Which I also sourced at the local big box home store. Should be able to get two of these side by side, and the bulk of my camping and cooking gear should fit in there. The area under the forward compartment should be able to accomodate most of my 'emergency' gear, which means the only "boxes" in my truck should be the cooler, the fridge (I use both), the water (a 6 gallon cubic jug) and maybe a 'dry food' box. This should allow for plenty of space for "stealth camping!"
Anyway, I just wanted to post this in casey anybody else was thinking of building a platform but worried about how to go about it. I'm pretty much the poster child for being all thumbs, so if I can do it, anyone can! My guess is that anybody could do this for a total cost of <$200, all materials included, and the only tools you'd really need would be a good saw and a screwdriver/drill. I used a table saw, which makes it easy to get nice, straight, long cuts, but I'll bet you could do it with a hand-held circular saw if you were careful.