Thanks for the help!
Just to give a bit more detail of what I intend to do...
I have a suitable length of extruded aluminium section (4"x3"x0.125"), which I'll machine out to accept a line of rectangular lights inside (4 or 6 depending on what I can get). There will need to be plates or covers on the back to give access. The ends I'll machine off at an angle and make suitable brackets to lock into the roof rails. Integral with the brackets will be pivots, so the main light bar can rotate 200 degrees, I have some small geared motors, one of which I'll use to remote-control the lights so I can point it where I like - including backwards for when reversing. I can't anodise something that big - just won't fit my baths, so I'll either have to polish the alloy or paint it. The thing is likely to get v. hot so I need to think about that.
Chris.