Cargo Bikes

Anyone know if trailers such as that below are DIY jobs (Bit of camber on the trailer wheels, or is it just weight) or can they be bought? Or maybe there are plans for them about?

I'm going to guess that anyone hoping to carry that much weight would ultimately require electrical assistance?

You wouldn't want to ride uphill very far, but I pulled 300kg in Max trike au nature. Slowly, and with a wide range of gears. Most of the rick-shaw guys do not have electric assist, and they pull 2 people.