This is not likely what you were thinking, but it is incredibly effective for theft deterrence:
Dark window tinting.
I have limousine-dark tinting from my passenger row, rearward. I worried that it would be a nuisance for visibility, but it really hasn't been. Night or day, you can see OUT from the inside, but people absolutely CANNOT see inside. Not even in bright daylight or darkness.
I keep mega-electronics on the backs of my front seats, and with the tinting, they are essentially hidden, along with anything I might have in back like rifles, computers, etc.
Dark tinting has other benefits:
-Keeps the truck FAR cooler in the heat of summer (no sun-beams blasting on the interior)
-Protects interior material from UV break-down
-You are not limited to hiding what fits below a rear cover. EVERYTHING back there becomes invisible, even if stacked to the ceiling.
I've never been a fan of dark tinting, because I think my brain always associated it with thuggish cars and bad attitudes (revealing some personal bias I guess), but wow. I love it now.
Also... It really does provide amazing privacy while driving, or sleeping in the back. Probably saved me a cell phone ticket or three when I pick up na incoming call, or set Google maps.
Here you can see the look. Front row is also tinted, but light. Second row and back is super dark.
I'll try to post an image of the view from inside to show visibility.
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