I built this property cart around the early 90's, I didn't build a suspension under it at the time because the jobs I built it for didn't require one, the 2 1/2" sq tube legs and 3/4" bolt/axles did fine.
When the grand kids started asking to get pulled around in it I decided it was time to put a suspension under it, that was about three years ago. First I burnt off the square tube legs, I still wanted to keep the deck high to make loading and unloading easier so I used a spacer between the trailer's frame and spring mounts, then I added the side bracing just to keep the wheel and axles from ever shifting.
I built the dump latch and the trailers jack stand myself, the jack's wheel came from a shopping cart. A friend machine operator who works down down at the local manufacturing plant bent the 1/8" fenders for me on his press.
The way I made the tailgate's slot was weld a piece of angle iron to the trailer's sides rear risers, it worked out great building them that way...they're strong, they will never get bent out of shape.
The cart is 6'X4', it's been handy cart to have around, my neighbors like it, they and asked if it was for sale as soon as they saw it but I said no. I don't think I would ever sell it for what I could get for it. They don't build them as strong as my cart. the frame and cross members are 3" channel iron, the sides and bottom wood are treated deck lumber.
Thanks for looking ...tackit.