I recommend the roll-n-lock. Both I and my father-in-law have one on our trucks. They are put together well and lock (with a key) to both sides of the tailgate, and it can be set in several positions (not just open or closed). Watch Craigslist. I got mine for $150 in great shape. They will set you back around $1k new.
2003 2500HD Crew Cab 4x4, S&B Dry CAI, AirDog 100, Sinister Diesel Sump, MBRP 4" Downpipe back exhaust, Edge CTS, EFI Tuning by Idaho Rob, Iron Cross winch bumper, Badlands 12K winch, Alpine GPS, Roll n Lock, 285/70/17 Cooper STT's on H2's.