Hi,
I agree. We used a roof bag on our last camping trip having lost the keys to our roof box (oops!) and the roof bars are an integral part of the safety of the bag.
The bars take the weight and the front and back straps have to be tight around the bars to stop it slipping.
I would be very worried that you would lose your bag and cause a trail of destruction behind you.
We recently changed our car so had to get a whole new bar and fitting kit. Whole thing on Amazon was just under £100 but worth every penny.
I just wouldn't risk it
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