When choosing a rooftop carrier, it is important to take into account the type of car you have and its size. A larger car with more roof space will be able to accommodate a bigger box, while a smaller vehicle may only be able to fit a small one. It is also worth considering what items you plan on carrying in your box and how much capacity you will need. For cars without pre-installed roof bars, you may also need to factor in the cost of purchasing these separately. Lastly, always check for compatibility between your car and the roof box before purchase as some boxes may not fit certain vehicle types or models.