I have a BMW 3-series (3-box shape: short roof w/ trunk) with OEM load bars
28 1/2 inches apart o.c. and I'm going on a car camping trip this weekend with 2 kids and too much gear to fit in the trunk so I got a Thule Tahoe
roof-top cargo bag.
It's 40 inches long and the instructions say 'It the distance between your crossbars is less than 40" drape the remaining length of the bag over rear crossbar'. I got the PDF of the instructions before I bought it but the picture is confusing and I really wasn't sure whether it would work without trying it. Now I've flopped it up there & it looks like it won't go flying off the roof, but it could very well shift around a lot while driving twisty mountain roads.
Food, stove, water jugs will go in the trunk. Only soft things on top: tents, sleeping bags, clothes.
It's 1/2 the price of a starter rigid cargo box and I'm not sure the mounting from one of those would work with my oversize crossbars anyway. At REI, I'd have to buy, then return if it doesn't fit. I've measured with dial calipers & looked at the boxes 'claws' & it's close, so close, but hard to tell for sure.
I know the bags aren't 100% waterproof and don't lock and that I'd have to find a place for a box when I'm not using it.