LOL - Thanks.
I built it from an old double baby stroller. It was a front-to-back rather than side-by-side set up, so the main frame of the trailer is made from the extra long "handlebar." The wheels are from a couple of old toddler bikes and the tow-bar is from a golf bag dolly.

I did a little write-up on it, back when I built it.
http://www.bikeforums.net/showthread.php?t=160351
I have since updated the hitch.
I used the metal portion from a caster, similar to this one:

With the caster mounted sideways and the wheel removed, a hitch-pin fits nicely through the hole where the axle used to be. It gives the trailer a full range of motion.
I attached the hitch to the bike using two U-bolts around the chainstay.