If you're just worried about people seeing the items in the back of your runner, then just slide out the tonneau cover. That way you can just use a regular old Igloo cooler if you wanted, use some commercially available oganizer trays to keep things neat, and then just slide the tonneau cover out so it's all hidden. And even if you dont want stuff to be seen by opening the lift gate, just access it through your rear window!
Just trying to save you alot of time and work for something you might find is not as user friendly as you envisioned. Like said, it looks a bit tall, and being deep in the cargo area, you'd never be able to see down inside it to find anything unless you crawled over the back seat, or crawled up into the cargo area some, kinda defeating it's whole purpose of being an easy access storage area that's incognito. And the idea of making it slide out would help with the access problem, but like said, anything you store between it and the rear hatch, would have to be moved out of the way first. PLUS, you'd need some pretty serious slides, and some serious attach points, to make it slide in and out properly and hold up to much use.
As for the cooler side, it'd be easy to make. Build the compartment, line it with 2" thick white foam board, then seal it off with two layers of fiberglass cloth, about 6oz plain weave, and some white gel-coat on top of that to make it pretty. or you could make a positive plug and mold the inner cavity from that, attach the foam to the outside of the fiberglass cooler cavity, and pop it in the opening of your storage box....
Just some things to think about....
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