I have a Blue Sky trailer that's really bulletproof. I bought it when I moved into my house, and it hauls pretty much anything I need from big bags of pet food to rugs to building materials. They're hard to find but some googling might turn up something. I've seen a few around town but only one in the LBS, which is the one I bought. It's made out of welded steel tubing and has nylon cart wheels with a marine-grade plywood floor. The hitch attaches to the seatpost via a loop of textile-reinforced rubber, like a slice of a car tire. This gives flexibility and durability with a simple attachment. I also use it as a garden trailer around the house, and I'm trying to train the dog to ride along in the cargo bed. It has waterproof side curtains and a waterproof rain fly so the load stays dry.

It was spendy, but it's definitely a car-replacer in terms of capacity and durability. I'll be mighty disappointed if I don't get at least a couple of decades of service out of it.

Cheers!