It doesn't look too hard. Making the hinge and the system for attaching it to the bike would be the toughest part. You could use one of the pivot points from a junky wal mart full suspension bike for the hinge and find a way to attach the thing and you're golden. (that's how I'm gonna do mine anyway)
I was planning to do this last year, before touring.
I was going to take an 'Adam's Trailabike' style U joint seatpost hitch, and make a boom that curved down, to a low C.O.G. platform...
I started designing, and pricing out materials..
Then bought a used BOB trailer from Craigslist, as it was cheaper and easier.
Too bad they don't sell those without the wheel for less $$. I'd get one that was frame only if I could use one of the extras I've got laying around.
Ride lots, have fun, skid often!
I built one using a bob hitch- you can buy the part from the BOB website for about $20, if I remember correctly. Then you can make any kind of one wheeler you want, specifically not that wimpy thing that appears barely to accomodate a backpack.