My flight bag is fairly standard (being a Cabair offering) and it fits into my Givi topbox easily. Mind you, it is huge, my girlfriend and I can get are lids in it. It does something strange to the aerodynamics as well, when the speedo is pointing somewhere between 10 and 11 o'clock on my VFR.
If you post this in Jet Blast you will get more replies, there have been a couple of bike threads there lately.