nice!
ive been thinking about the best way to do a trailer for a while now, theres only so much i can carry on my small rear carrier (makes more frequent shopping trips necessary which is both a pain and bad for economy on my motorised bike lol!)
i have a pair of wheels similar to yours laying around Sven7
http://i40.tinypic.com/aw9j4z.jpg
how do they do at speed? obviously my motorised bike will go a bit quicker than your pedal only one but do they show any signs of stress if your really moving like down a hill or something?
im thinking a simple 1"x1" steel frame and hitch for plenty of strength and durability and expanded aluminium mesh for the base to make tying stuff down super easy