I use 2 Jeep bags with wheels and handles to fit long ways then 2 small ruck sacks.
Or on on cheep ryanair its 2 wheelie hand luggage bags and 1 jeep bag.
Tresspass do one as well that fits fine, forget about bags you can not wheel at the air port, and they get damaged,
I fly 5 times a year on holidays so sod custom fit ones. Enough for summer or colder spring and autumn holidays, and yes 2 weeks ski hols as well. This was one of the key selling points for the z4, The first one I got the stealer to drop the car off at home so I can try the luggage (infact I never went to the dealer actual place to pourchase did it over the phone and they came and swapped) and yes I took the luggage to the last dealer to try the new z4 for size.
Only issue is you cant drop the top, I could in the old one with the same luggage but got sunburn on the way to the airport stuck on the m25 to gatwick, looked a bit stupid arriving at stlucia already red.
As the Admiral suggested - But I'm using one suitcase across the bulkhead end of the boot and a soft bag / solid base with wheels and a pull out handle (Wengers on offer at Tescos) across the back of the boot - It all fits in like it was made for it and I only had to spend £26 . The roadster bag solution looks the biz but at £180 it seemed to be a waste of cash -
Summary:- One Tripp Case £59 and One Wenger £26 ! Boot space filled with luggage and I can drive top down