I'm looking to add a cargo box to the top of my RX350. Any suggestions of the best box to add would be appreciated.

 

Welcome to CL, rdstr!

Yakima makes some nice boxes and has a great deal of experience with them. There are some fabric bags that might work too - depending on what you intend to put up there.

At one time there was a warning not to exceed 70lbs on those roof rails - and that's probably still a good idea. You don't want to get too much weight up high that can upset handling.

I once used a couple of capped 8" PVC pipes 9 feet in length lashed securely to the rails to carry rolls of seamless paper to a photo shoot without having to share the cabin with them. They were fairly light and the PVC worked OK, I might use them again for any long objects I don't want to get wet or bug splattered.

 

Looking to store a couple of sets of clubs on the box.

 

I bought a Thule Cargo bag and it is excellent for carrying gear and keeping it dry. The weight limitation for the roof rack on the RX350 is 165 lbs (75 Kg). This will not raise the center of gravity to affect handling. The cargo bag I bought is made out of heavy duty canvas/nylon with heavy duty zippers and can store in it's own storage bag which can be used to stuff gear into. The cargo boxes are bulky and you need more room to store them.Thule is another maker of excellent cargo bags, boxes and carriers. If you go with a bag I place a roll of that waffle foam for lining shelves between the roof and the bottom of the bag to avoid any scratching.