Couldn't you just buy pvc elbows instead of bending straight pipe? I would think it would be alot easier, and the elbows and connectors are pretty cheap (the job I just left is in a home improvement store).
If you wanted a light-duty one, you could probably use the super-inexpensive (but very durable) white plastic pvc pipe, too. Pipe cement (and the proper connector pieces) would hold the whole thing together. You could even use copper tubing and elbows if you wanted; I think those are cheaper than the heavier metal pvc, but could hold quite a load, especially built in a frame like that.
I have a couple of wheels from a dirt bike (my parts bike); I may get some of that copper pipe and try it out. I have plans for a simlilar one that I downloaded in PDF; I'm not sure but it may have been from the Bikes at Work site, or certainly a similar site. I'd have to study the hitches, though; my idea was to attach it to the back of the rack part of my saddlebag-type rear baskets. I'm not sure how steady I'd be pulling a heavy load either way, but one of these days I may get the chance to try it
I've even seen a picture of one that someone built; it was a long flatbed trailer, with a recycled ironing board (just the metal frame, no fabric or legs) as the bed, it was pretty cool!
I need to build my own so that it's kind of longish, but narrow enough to go down a sidewalk; I deliver newspapers in the wee hours of the morning, and the baskets just don't hold all of the papers for two routes, or I could bike the whole thing from my house and leave the car at home. I'm up and down driveways and sidewalks the entire time. It would probably be bad if one of the trailer wheels bumped up in the grass and dumped my papers all over somebody's front lawn.
Even so, I would probably only use it on Sundays, if I get the papers early enough, because they're just too big to all fit in my baskets, even one route's worth. I'm still waiting on that nifty front-and-back newspaper carrier vest for the other six days of the week
