Carpy had one for sale...
I was going to buy it, but my money ran out.
I don't like top boxes at all. Don't like panniers much either. But, since nursey Mrs Jools is still not completely recovered from the herniated disc she sustained lifting old ladies into bed, she can't do much more than an hour on the bike at a time. I was going to buy it more as a backrest than anything else for when we want a long weekend somewhere, plus it gives a bit of extra capacity.
For day-to-day use I wouldn't use it. In fact my panniers are only trotted out twice a year or so, and I take the whole kit off (carriers and all) between times. Only takes half an hour to put em back on. I imagine the top box would be the same kind of deal.