I found this http://www.ehow.com/way_5838858_home...eau-cover.html
also this http://www.ehow.com/how_7295304_diy-...eau-cover.html
i personally would not drill holes into the TOP of the bed rails, i might consider drilling into the inner edge of the bed rail.
I wouldn't hesitate to use a spring loaded J hook
, its a hook in the shape of a J, with a spring at the top of the straight part, you push down on it compressing the spring, then rotate the whole thing, until it catches under the bed rail, to release you compress it again & rotate it, letting the unit pop up because the spring is no longer compressed. I forget what they are called