I got a BOB Yak cargo specific trailer in the mid 1990s and the thing is still going and going and going. If you are going to use the trailer to haul cargo and not little people, I would highly recommend it over a conversion. It has much better handling, since it attaches on both sides at the rear dropouts and rides on the mid-line of the bike, so it doesn't pull one way or another.
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You can even attach the child trailer (without child, of course) to the BOB, for even more capacity and even less speed
(although you probably void every single applicable warranty when you do):
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On that note, we actually recently just bought a purpose built cargo bike. It's an Urban Arrow, and more aircraft carrier than bike, but it does do its job.