Here's my HF folding trailer with supposedly 1,800 lbs of dirt (1 yard) in it. It probably took me 20hrs to assemble, repack bearings, build floor and sideboards, stain the wood, and wire electrical. It looks simple just looking at it, but it probably has 80 bolts holding it together. I don't see it as necessary to weld it.
I might have only paid around $200 for this trailer, but if I did it again, I'd probably just pay extra for a trailer already assembled considering the amount of time required to put it together.
The stake pockets are designed for 2x4s, and sits flush with the 4'x8' floor. This means if you attach sideboards to the stakes, you will have less than 4'x8' of floor space (by the thickness of the sideboards) and can no longer place 4'x8' materials inside.
I used 1x4 stakes and cut the sideboards with tabs that when attached to the stakes, fit inside the stake pockets. This allows a full 4'x8' floor.
Since these photos, I have installed the metal corner brackets that allow the boards to interlock, so I don't have to use the ratchet strap to hold it together.
I really enjoy the folding aspect since it takes up very little space when folded up, and allowed me to store it inside my garage. Most people will need a hand to fold or unfold it because it's a bit awkward.
