The cap is fiberglass, not metal. the top 'slats' are just minute depressions, the roof has some curve but would hold a magnetic sheet.
also, rear visibility is not an issue. I usually leave the rear window blacked out to conceal all the gear I store in there, and the shelf in front of the cab backlight is usually packed with crates anyway. It does mean there's not much I can do about the big side mirrors - I need them.
Pics of my truck:
looking down on the top:
here you can see the gentle radius on the back of the cab, and the scoop-like effect of the front of the cap. it just seems like it might pull some air into the low pressure zone under the truck.
