This is how I did it...
A caveat: actual measurements are lacking because many trucks come in many different sizes... and to be honest, I actually did a lot of eyeballing for this one; ahaHAH! :D
Step 1: Materials
-Phillips head driver or bit
My side panels are 14" tall, and 8" deep (to reach just the inside edge of my wheel well). I used a jigsaw to remove the cutouts on the bottom back edge of the shelf. This was done to compensate for the wheel well protruding beyond 30" (which is the measure of my bed slats).
Step 2: Measure the Storage Area
Next I started tracing the size of my slats into the area they would end up. I did this for both side-panels.
Then, I drilled where I want the screws to be.
I screwed one of the slats in place to make sure the box was straight enough. I continued this process with the other slats; making sure they were straight.
For the back of the bin, I used just one slat and made the top edge sit plumb with where the top would sit. The inside bottom will be left open, to let whatever is inside breathe, and so if dust and particles build up over time, I can just brush it out (I'd rather not let things like random tools and such collect).
Step 3: Finishing
Place the lid onto the bin, and inspect your work.
Step 4: Before and After
Step 5: After Thoughts.
The type of wood I used is pine. I wanted something light to help keep the MPGs optimal, since I may carry a few more things.
I used a lot of screws because the bin is easy to move around, making the bin stronger than it needs to be gives me the peace of mind that it'll withstand the jarring effects from rough road, and my bumping into it.
The material is easy to customize further. If you're so inclined, you can stain it whatever color you want. You can even use a soldering iron to burn your custom design.
Since the materials are reused wood, they had imperfections like staples and nicks, and some of the edges could use a bit of sanding. I chose to leave some staples in (but made sure they were pushed in so as not to snag anything). Personally, I like the imperfections.
Even though the majority of the weight causes the bin to lean onto whatever wall by which it sits (note the cutouts on the bottom rear edge of the side panels), it may still fall forward (away from the wall). To prevent this, you can mount hooks from the back of the side panels. The hooks would tuck under the bottom edge of your truck bed rails, thus preventing your bin from falling over during sharp turns. Another thing: add some Velcro strips (the loop or plastic side) to the bottom of the bin where the feet and the bin itself touches the bed rug. It'll help to further keep the bin in place;)!