I currently have an older Yakima RocketBox that I used on my Toyota Sienna. I'll probably get a new box of some sort for the Highlander. Some comments about boxes.
1. Come up with some system when using boxes or bikes on top of the car to keep from driving into the garage. My system is that I have a step ladder in the garage that I set out into my parking place anytime I have bikes or a cargo box on the car to remind me when returning late at night and tired not to drive into the garage.
2. Two main issues with fit are going to be rear hatch clearance and bar spacings. The bar spacing on the Highlander are unfortunately fixed at what....27" or so? This could possibly affect which boxes are usable. Measure your bars from center to center and the boxes to be sure. Also how far the rear of the box extends will affect the ability of the lift gate to open all the way up. On my Sienna the Rocket Box is very long and will prevent the lift gate from opening completely. Not really a problem as it is a manual lift gate but could be more of an issue with the automatic lift gates on the new Highlanders. I'm not sure what happens to the power lift gate if it can't open all the way.
3. Weight capacity will be determined by the crossbars and crossbar attachment points. These boxes will hold enormous amounts of weight, it is the crossbar you need to worry about. I suspect it is actually the crossbar attachment point that actually determines capacity as that seems to be the weak link in the system.
4. If you park on the street, pay attention to which side the box opens. You definitely want a box that opens on the passenger side if you normally park on city streets because you live in an apartment.
5. Buy a small folding step stool and stash it in your car. These beasts are too high to deal with unless you are in the NBA.
Great way to have extra storage and I put my kids stroller and bags up there when we have the third row seats up
