I think if you all take a look at the cartbike, you will get some great ideas. now this is not a trailer, but more of a tadpole bike. But, you can take the exact same shopping cart design, turn it around so it faces backward, drill and mount an "S" shaped pipe from the bottom of the cart that rises to the seatpost, then use tightwad's "chunk of car tire" hitch with a big bolt that goes throught the end of the "S" pipe which has been flattened vertically. you can make this entire trailer with NO welding. beautiful!

http://www.bicyclelaneindustries.com/bli/cartbike/

regards,
flashdancejay