Low Profile Roof Box

Does that mean you already have a roof rack/crossbars?

 

There are rooftop cargo bags available that don't require a roof rack, but you need to be careful not to put too much weight on your roof, or you could damage it. Also, the straps that go into the passenger compartment compress the weather seals so that any heavy rain encountered can leak into the car.

https://a.co/d/i6dUzzS

 

I definitely agree, but I wasn't sure if you were looking for a less expensive alternative.

 

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I find standard roof boxes too bulky to store it in the garage afterthe holidays, so I use a windsurf sail bag, which can be folded and stored in a cupboard in winter, aerodinamically wise it's long and narrow, if you hit something like a garage roof it' s soft, so no damage if you don't hot it too hard. The bag is not completely waterproof though.
I also use original toyota roof racks

 

IMHO; anything you throw up there will impact your aerodynamics and cost you some mpg.
I'd like to suggest an alternative; one of those trailer hitch boxes. You'll need to be-careful driving around with it, because it shifts weight back - but it should be fine if you don't overload it. The bonus is that it can easily be removed when not needed, since it sound like this is an occasional thing.

Hope this helps....

 

I can confirm that in my experience the hitch mount is definetly the best aerodynamic option.

I usually secure bags on a hitch bike rack with tension straps, but the bikehas to stay at home