Firstly check the car is the class that you booked - we suggest you always take a copy of the details you were shown when you booked the car. If the car rental company suggest they do not have the car you booked, ask for an upgrade rather than trying to squeeze into a smaller vehicle.
Even if they don't have a suitable alternative it's possible that another nearby car hire office has, and it can be delivered to you.
If the problem is that you have booked a car which is too small, then you have the alternative of upgrading (probably at your cost) or asking if they have a roof-box or a roof-rack. If not, a local car depot (the local equivalent of Halfords) might sell a cheap model - probably for less than the cost of hiring one from a car hire firm.
Be very careful if you are fitting these though and check whether the car hire firm are happy for you to do so - car hire firms exclude damage to the roof from their insurance cover as they class it as negligence.
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If you have any queries do call a member of the MoneyMaxim car hire insurance team.