as these are made of plastic and usually in black colour wise, its surprising just how hot these get!!! and particularly of late. i would rather put a strap around it to makesure than have to try and pick everything up when/if it bursts open on a busy road!!!
when the boxes get hot the plastic becomes a bit pliable and thats when the locks could move together with the shaking they get when on the road
We started using a roof box this year and on the few occasions we have been away we have added two bungee cords just to secure it further, mainly for peace of mind. Whilst driving down the M5 to South Devon last week we see at least three cars that had scattered their belongings cos the roof box had opened. This in turn caused mental hold-ups on the motorway, traffic was horrendous and could have been avoided if they had strapped it down, funny thing was I saw one car lose an ironing board!!!!!!
So yes "strap it down", for the sake of a couple of quid. You won't be sorry.
Ordinary luggage straps will do, as long as they go completly round the roof box. As you are borrowing this, and perhaps it is your first time using one, REMEMBER that it is on when you go into car parks etc with barriers. I know from costly experience!! Was the other half driving not me!