Cheapest cargo trailer on the market
Sometimes, it just doesn't pay to do it yourself with scraps and leftover pieces, especially if the results aren't exactly safe. At the same time, few of us have a lot of money to throw around, and most of the cargo trailers on the market seem to have solid gold parts somewhere from the prices they are charging. In addition, the cheaper "kid carts" aren't very sturdy and don't work well for big dogs or cargo.
I took a risk and bought an Aosom trailer. They come in 3 sizes, and are available on Ebay for about $100-130 depending on size with pretty reasonable shipping. (I actually bought mine from aosom.com, but they have been out-of-stock for months & didn't do anything to inspire massive amounts of customer loyalty anyhow.) These cargo trailers are really GREAT. They are stable, have a solid metal bottom so there are no worries about that giving way, and have reasonably sturdy sides made out of nylon canvas. Supposedly they can be towed by a motorcycle as well (though I'd not want to take them at 75 mph) so I have no worries about putting it behind a motorized bike either.
They aren't fancy, nobody has ever heard of the brand, but I don't need frills--I need cheap, sturdy & reliable. We're happy enough with our small one (which carries a 75 lb. dog around) that we're going to buy an additional trailer in large. Plenty of weight capacity means you can haul about anything you can pack into it.