I chose a Tech-7 26 litre top box, since it's cheap, and lasts well (i already have the 48 litre one. I can fit the missus in it!). I mean, i could buy two of for the cost of a givie, and everthings included. Cost; M&P Direct £29.99 or from http://www.vcustoms.co.uk for £25 with zxr400oc discount!
Buy a spare passenger seat from someone (racers often take them off), plonk your top box mounting plate on top, and pick 4 good spot to drill through. Do so. Now either get some longer bolts and bolt the plate on, or cheat and use large plastic zip-ties. Pop the pillion seat back on the bike, attach top box and voila, instant quick release luggage. since most covers won't fit over the top box too, just unlock pillion seat before covering, and leave it attached to bottom of the box.
This layout doesn't buggar about with your aerodynamics too much (its just a rather large seat hump...) nor your weight distribution as much as a large rack mounted box would.
{Will add picture on bike in the morning.}