I have seen them, and I like them, I had thoughts of making one for my Sport Trac, but then I don't haul that much in bed to justify the effort. My other concern would be how well they would work in a composite plastic bed??
Seems pretty simple to just make one of those considering you can find glide rails long enough. Mount glide rails to the bed, 3/4" MDF or marine-grade plywood from HD or Lowe's mounted to the rails. How much do they cost?