Ah, I see. That looks like a small 'executive' briefcase but that could be because it is in a cavernous black hole...
My 'regular' briefcase was (still is but isn't in current use) 500mm x 380mm x 140mm.
However, either way, the pain of having to live with a bike as big as my car means that your topbox is a total non-starter and anyway, it really isn't the sort of thing I would want to carry through an international airport every couple of days just to get my laptop, files, fresh shirts, socks and duty-free on board.