I've used a few different trailers over the years and I'm in the process of developing a (welded) farm cart that also has a tongue for hauling with a bike (farmhandcarts.com). If you're looking at something that doesn't require welding check out Bluesky trailer hitches, which use a piece of rubber strapping and a pin to mount to the seat post. Human Powered Machines' Hauler trailer uses a great quick disconnect ball joint that can be purchased. You still need to figure a way to clamp it onto the bike, but it's a great set up. Hitching the trailer at the seatpost isn't optimal for bike geometry in theory, but in practice I've found it to work fine, and it makes it easier to hitch, as well as easier to move the trailer around when it's not hitched to the bike. Without welding the frame I think that following the garden cart concept of bent thin wall EMT and chanel bolted to plywood is probably your best bet.