One of the simplest and cheapest hitches i've ever seen-a thin metal cover for an electrical junction box, about the size of a lightswitch cover. Nick the top and bottom with a file at the center mark, fold in half and seal with some JB Cold Weld. After it's cured, drill a hole to attach to the wheel bolt and a smaller hole beneath for a bolt-in Eye hook. Use a clip-on hook to attach the trailer arm to the Eye and you're done! Cost-about $2 for the cover, bolts and hooks.
I wish I could say it was my idea, but I ripped it off this website:
Instructables - Make, How To, and DIY
I've never seen more Bike Trailer designs-and different mounting methods-before in my life!