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).