I have built a few trailers in my time.
The latest one is a log hauling trailer.
I used the coil suspension from an old caravan, to save reinventing the wheel, and from that I added a box section spine and an angle iron frame. The frame was made to hold two 4'x3' folding pallet crates while the spine became a telescopic drawbar.
The two scaffold pole legs at the back can be changed for longer poles with a cross bar that I use with a chain or cable hoist to lift logs into the trailer. There is also a roller across the back so that the logs can roll on as they are loaded.
I also have a winch on the back of my car that I can use through the trailer to pull a load up.
The folding box sides means I can have one big box, two segregated boxes, one box and a flat bed or a large flat bed. The crates are also removable to leave a skeletal frame.
The draw bar extends giving me 8' to 15' of trailer.
The cross beam at the hitch will eventually have a pair of pull out extensions with jack up legs and then a crane will be added. I bought this electro-hydraulic crane a while ago just for this purpose.
I might be able to dig out some build photos if you want.