Is there a major car rental company in the U.S. that also rents roof top cargo box?

In my experience when rental car companies rent you anything that you might want to add to a car (extended roofrack, car seat, etc) the price per day is a large fraction of the total price of the item. For example $20/day for something that costs $35. I suppose this is to cover all the paperwork and hassle of making sure you return the item with the car, storing the item somewhere when nobody is renting it, and dealing with potential liability around the way the item is attached to the car.

As a result your best bet is to buy the item outright, and either resell it, give it to a charity, or leave it at the side of the road for someone to just take when you're done. It can be argued that it's inconvenient to drive your just-rented car to some kind of store to get the accessory before carrying on with whatever you're renting the car for, and I'm not going to argue with that, but unless you're renting for just one or two days, the extra cost is going to wildly outweigh that inconvenience.

Without more details (the length of your trip, the reason you want the box, the constraints that make it very difficult to get the box anywhere other than the car rental company) you're unlikely to get a better answer than this: drive your rental car to a store that sells roof top cargo boxes, and buy the cheapest you can.