I have found that the weight distribution and mounting points on most roof racks pretty much make it useful for carrying your lunch. I do know the purpose-built ones are great, like the ones mentioned above. If you look at the load rating on most of them they are pretty conservative. Back in 1986 I took the biggest carrier Sears offered, approx 6'x3'x15", and actually bolted it to the roof of my Aerostar. I used nine 1/2" or 5/8" bolts with 2" washers sandwiching the sheet metal. I didn't trust the screws that came with the carrier. That allowed me to load it up well beyond the 75 lbs the manufacturer recommended. I did this for my move to Alaska when I was in the Air Force. I needed baby supplies, a place for the rifles, dog food and sundry other items. Later I used the bolts to mount a dual antenna rack for a CB. Well I've taken up too much of your time. I'll stop.