If you have basic steel fabricating and welding skills why not build a licensed Carla Cargo Clone:
Bauanleitung Carla Cargo Crowd ? postfossil mobil It is made mainly from ⅝"& ¾ 16 gauge/1.6mm thick mild steel box section. It is economical to build along with being both reasonable light and strong, and the third "tricycle wheel will give additional stability when loaded and is a good starting point for an electric drive conversion.
However, you may need to use google translator because it is written in German.
I currently have a friend a few miles away converting the Carla cargo trailer frame I recently built for him to a Romany style bicycle touring caravan like the one in this photo:
