For those E53 without roof rails, there are BMW EBA (Einbauanleitung, i.e. installation instructions) directions for this and a factory parts set as well. One number covers all E53 cars. It does involve patterns you lay on your car's roof, cutting sheet metal and priming and painting to avoid rust.

I had considered this but in the years of BMW ownership (with roof rails) ended up putting my "stuff" inside the car (X5 or touring) with the seats down. Ski bags help a lot as well.

If someone wants details, contact me. The kit is NLA but you can find old stock at some dealers (I found some in Europe, also in the US). Don't spend more than $100 for a COMPLETE used set with hardware.