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).