I built a simple box that slides on my bed liner. I thought I might have to add either wheels or nylon sliders underneath but it actually slides pretty easy. I didn't build it full width, so that I can toss our outdoor carpet alongside, and also a charcoal container and milk crate for our extra propane
tank. The bottom of the box is slim 1/8" hardboard so it doesn't waste much height at all.
The items in the box are able to fit under the platform I added at the bed height, and since the platform isn't full length, I can put overheight items in the section closest to the tailgate.
The space at the top of our shell seemed wasted, so I built a platform with PVC and closet rods that swings up out of the way and is exactly the right size to hold our folding table on the left side, and extra solar panel
on the right side. When we need one or the other, I just lower it a couple inches, slide the table or panel out, and then put it back up. I even through our PFD's up there out of the way for our canoe.
When you have a short bed like I have on our Dakota Quad Cab, you have to use every inch you can get! :-)