That Cabela's product works pretty well. Some tailgates are easier to seal than others. Spend the time to test fit some small pieces all the way around to get the right layout, then clean the metal well with rubbing alcohol before you start the final installation.
For around the shell, I purchased some custom rubber P-bulb material from an industrial rubber supplier. Much tighter seal than the rubber foam, and I'm sure it will last much longer. My older Flippac doesn't fit perfectly on my newer truck, and the bulb fills the gaps nicely and seals everything up. Check around the 'net. There are lots of sources for rubber seals. Pretty much anything can be sealed on with some Pliobond.