Truxedo -Truxport
I use a Truxedo brand "Truxport" cover, this type sets on top of the bed rails, doesn't take up any space on the inside of the box.
It's been on the truck for 5+ years, sets outside 365 days a year and no problems.
Works nice, rolls up to the cab for full access to the bed if needed.
Here's where I purchased mine from:
http://www.truckaccessorizer.com/
They carry several brands & types.