One of our volunteers has designed this bike trailer, using a Royal Mail bicycle's luggage rack as the main component.
Feedback: We've had feedback that the caster used for the hitching point isn't strong enough for the purpose. Also the trailer isn't as stable as we'd like so a two-rear-wheel solution is currently in the works.
List of other places to find trailer designs:
Bamboo trailer - Carry Freedom
"Kyoto" Utility Trike (aka Drumtrike) - Mark Rehder
Single wheel trailer - French site, UK translation