Xfer switch question

Hi, I am looking to get a transfer switch for a portable generator. I would like to get one that is like a sub panel so I can add or change different breakers as I need. I am looking to get about a 50A switch and wire it to a twist lock plug that will be mounted outside my house. Anyone have any suggestions?

Like this:
http://www.electricgeneratorsdirect.com/images.php?products_id=2830

Here is another source.

http://www.squared.com/us/applications/residential.nsf/LookupFiles/GeneratorPanels2005.pdf/$file/GeneratorPanels2005.pdf

Why not just switch the entire main panel with this - interlockkit.com

-rick