Here's a pic of the official Honda rack. Yup, that takes a couple of mpg. It look like it mounts to the roof drip rail. Probably uses torx of square bit drivers. Here are the assembly instructions. You'll need new roof moldings. Just removing the crossbars will give you most of the gain and will be the least PITA.
http://www.handa-accessories.com/ridgeline/roofrack.pdf
