Rooftop cargo boxes are an easy way to haul what won't fit in your trunk. The largest among those we recently tested offers 20 cubic feet of room. Many of the latest models are reasonably easy to attach, load, unload and remove.

There are three basic types of cargo boxes, each of which requires a roof rack -- an additional cost of $200 to $300. To pick the right box, begin with what you want to carry:

* Wide Sport Boxes. These are usually around 7 feet long by 2 1/2 feet wide -- long enough for skis and some kayak paddles, yet wide enough for other gear. They're the highest priced (usually $350 to $600) and the heaviest (about 40 pounds) overall. Their length is likely to prevent the rear hatch or, on some vehicles, the trunk from fully opening.

* Narrow Sport Boxes. These are also around 7 feet long -- restricting hatch and possibly trunk opening -- but are only about 1 1/2 feet across, leaving room on the roof rack for a bike or a kayak. They're relatively inexpensive (typically $200 to $350) and usually weigh about 30 pounds.

* Luggage Boxes. Most are about 5 feet long by 3 feet wide and weigh around 30 pounds. They're useful for hauling suitcases and other items that are bulky, not long. They might prevent a vehicle's rear hatch from fully opening. Typical cost: Between $100 and $300.

Most cargo boxes can hold up to 110 pounds. But some vehicles are rated to carry as little as 100 pounds on their roofs, including cargo, cargo box and the roof rack that supports them. Check your car's manual for the rating. Overloading your roof can make your vehicle top-heavy, raising its center of gravity, and that can increase the risk of a rollover for many sport-utility vehicles, which are already rollover prone.

Once you've settled on a carrier and determined the weight your vehicle can safely carry, look for these features that can make some cargo boxes easier to use than others:

* Boxes that open from either side eliminate the need to reach across the box when retrieving cargo, as you must with boxes that open from just one side. While that isn't a concern with narrow sport boxes, it is with wider models. Boxes hinged in the front are less convenient to latch and unlatch than side-opening models, but they still allow you to reach all the cargo.

* Installation involves clamping the bottom half of the box to a roof rack. Quick-release hardware attached to the cargo box makes installing and removing the box easier.

* Internal screws, bolts or rivets that are covered won't scratch or snag cargo. Tie-down straps inside the box can help prevent cargo from shifting.

In our tests, three Thule models stood out:

Among wide sport boxes, the Thule Classic 601 ($400) scored highest. The slightly smaller, slightly lighter Thule Vision 663 also received an "Excellent" rating, but it costs $200 more.

Ease of use and easy installation helped the lightweight Thule Frontier 668 ($250) top the narrow sport boxes. The second-ranked Yakima SpaceBooster 7100 ($250) is shallower and not as easy to use, but it holds as much and is a worthy choice.

Among luggage boxes, the high-rated Thule Excursion 667 ($240) is our top choice. It's relatively lightweight, but has a rather small capacity. More capacious (though less user-friendly) is the Sears 7200/Sport 20-SV ($175).

Whichever box you buy, put moisture-sensitive cargo in plastic bags: None of the boxes we tested was totally waterproof.

(c)2001, Consumers Union Inc.