Rather than trying to install a factory roof rack, which requires drilling,
I recommend either a Thule or Yakima roof rack system.  Both offer excellent
quality products and are sold at sporting goods stores, outdoor specialty
stores like REI or Erehwon, and bike stores.

We have a Thule Evolution 1200 rooftop box that attaches to our factory rack
but would obviously attach to a Thule rack as well.  The aerodynamic shape
is very quiet and has much less drag than the boxier rooftop box we had
before.  The Thule box is very waterproof, I've never had any water get in
during severe thunderstorms and in snowstorms.

The biggest drawback to a rooftop cargo system is having to get stuff into
the box.  The higher the vehicle, the harder it is to get stuff in, and the
additional drag of having something up there.

The Thule Terrapin hitch box is another alternative.  It attaches to a 2
inch receiver hitch and because it sits behind the vehicle, there is no
additional drag to speak of.  I believe the trailer hitch for the RX is
1-3/4" so you would need an adapter for it to fit.  Places that install
trailer hitches might have the adapter available.