I have used the Weather Guard boxes before, they were pretty good. Haven't had one for a few years, so can't comment on recent quality.
http://www.weatherguard.com/lo.htm
I have looked at the Higyway Products stuff before, and looks to be very good, but have not actually owned.
http://www.highwayproducts.com/saddle.htm
Most of the boxes you see in retail stores are pretty light duty, so I think you want to explore options such as I have listed here. You usually get what you pay for, sometimes less.
One of my favorite quotes is in order here:
"It's unwise to pay too much, but it is worse to pay too little. When you pay too much, you lose a little money--That is all. When you pay too little, you sometimes lose everything, because the thing you bought was incapable of doing the thing it was bought to do. "----John Ruskin, Late 1800's
Shelby
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2001. 5 2500 QC LB 4WD, ETH (Built 09-10-00), DEE, 3. 54 gears, Laramie SLT, Camper Special, Trailer Tow, Travel Convenience Group, Power Driver Seat, Alarm System, Remote Keyless Entry, Fog Lamps, Cab Clearance Lamps, HD Transfer Case, Bright White with a Gray Interior. Built 09-15-00, took delivery on 10-06-00, 3873 miles as of 01-04-01. Custom aluminum work body / flat bed. Coming soon: E-Brake and gauges.
[This message has been edited by Surveyor Shelby (edited 01-11-2001). ]