For a rear seat that mounts to an existing rack, I recommend either the Bobike Maxi City or the Yepp Maxi Easy Fit. Since the price is so much lower, I recommend ordering from Holland Bike Shop; even with a $40 shipping charge, the total price is more than $50 less than the American distributors!:
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A point in favor of the Yepp is that if you did end up with a cargo bike, both the Yuba Mundo and the Xtracycle Edgerunner platforms are designed to work with this seat (the pre-drilled deck holes are aligned with the holes for the Easy Fit Adapter. The Bobike adapter is a little lower profile, so it would keep the seat lower.