hi,
i usually just tie my tent across the back seat against the top box, but if your two up that rules that out, ....
i (and a couple of mate ) have just bought some non slip matting from the pound shop and placed that on the top of the top box and then bungeed the tent on, never budged at all and used it like that in all weathers, thats the cheapest option.
i have seen small racks that bolt directly to the lid of your top box to attatch extra bits to..