Clamping a bar to an existing roof rack ain't exactly rocket science. It's the strut channel that does all the heavy lifting in this Instructable, I'm just here to type up the notes. If you are already familiar with strut materials skip ahead to the Step 1.
If you're not sure what strut is just look up in a grocery store and you'll see it. Strut is that stuff suspended from a ceiling by rods to support electrical conduit, refrigeration lines, plumbing, etc. in commercial buildings. Think of an Erector Set in real world scale. While it ain't pretty, strut channel has a lot going for it. (How I love thee, let me count the ways.)
Inexpensive and readily available
Galvanized steel for strength and durability
C- channel in cross section for added rigidity
Predrilled holes create multiple points of attachment
Flat sided for easy connections
Easily drilled with power drill
Easily cut with power tools - I haven't tried a hacksaw on it yet
Matched perfectly with special connecting hardware
Did I mention inexpensive?
Step 1: Gather your materials and tools
Strut channel (comes in 10 foot lengths)
U-bolts (two per cross bar)
Metal cutter of choice - reciprocating saw, hack saw, cutting disc for drill, etc.
Metal file for smoothing cut ends
Something to pad the contact points on the car's existing rails - I used heat shrink tube and inner tube rubber.
Something to cap the bar ends (so no one gets hurt).
Non-permanent Loctite or double up on nuts in key locations.
Any custom attachments you need to fit your toys.