Roof top boxes - help please

[CarGirl]

I am a novice roof top box person - I want to buy one - 2nd hand if possible I have an old M reg expace with a roof rack on it. What type do I buy, are there special types for diff roof racks etc.

What do I need to know etc?

look at the Thule website. They make top boxes and have a guide by car make on what you need. you don't have to buy from them but it will help you work it out. you probably need roof bars (which attach to your car) and then the box attaches to them.

thank you!