The build was simple, a rectangular frame with a rounded nose. I used the rounded front from a floor buffer which set the overall width. It ended up being a fine size. My original build for the suspension was both too complicated and too heavy. If you are making one use the axle set up I show in this video, not the one in my first attempt. Either way the setup works and is durable. Just make sure when the trailer is sitting level with no load on it you have the trailing arms and lateral bar parallel with the frame. This is important to keep the axle in line when the going gets rough.
We pounded out over 500 miles on the original layout, much of that on dirt roads. It has been one of the most enjoyable builds and the time spent with my son has been priceless.
Springs! I used part number 1692K45 but I think the 3 inch would have been better.
Ball Joints! I used the 1/2x20 for the trailing arms and 3/8x24 for the lateral bar.
Threaded Rod! They have stronger stuff but I don't see why you would need it. You can also use their stainless but I just painted mine. I bought two 3 foot sticks of 1/2 20 thread (you have to match the thread to the ball joints you find, whatever they are) I used one stick for the axle and another cut for the trailing arms. Starting over I would use 3/8 for the trailing arms, 1/2 is overkill. I would buy one 3 foot 1/2x20, one 3 foot 3/8x24 and one 2 foot 3/8x24.
WHEELS! I got these from Northern Tool. You could source your own if you needed to. I used 16 inch but have thought that 12 would have been fine and kept the cart lower to the ground. Starting over I would go with the 12's unless you are planning on some serious off road, but if you are remember it will tip over.