I know these posts are from the summer, but having built a capable off road trailer I can offer a couple of thoughts. I must say I made the mistake many first time builders make which is I overbuilt and probably made the trailer a few hundred pounds heavier than it needed to be. Having said that a strong frame is a good place to start. Mine is built out of thick walled 2x2 square tubing. It uses a 3500' leaf spring axle. To improve the ride the springs were exchanged for lighter weight springs, if memory serves they are 2500' springs. Leaf springs were chosen because they are simple and easy to repair. Also I keep a spare inside the trailer in a small compartment. 1/4 plywood on the outside and 3/8 on the inside. A different method might offer some weight savings here, maybe an aluminum skin and foam inside. Once the trailer had the walls put on I took it off road down a fairly difficult trail. First incarnation it had a 48" axle which made the trailer tippy off road especially considering the trail I first used. I was going a little 2 fast on a rocky off camber section, by to fast I mean approx 2 mph
and the trailer tipped over. My buddy behind me has a 54 Willy,s truck with a winch and he quickly lined up and we winched the trailer back to its wheels. The next part is embaressing but I did not hook the line to trailer very well and when we got it lifted about two feet up the hook slipped off and the trailer fell back on it,s side, not once but 2 more times. Much damage was expected but since it was a slow speed tip over and was not dragged at all on its side other than scratches on the paint, no damage. Any way the point here is light weight is good but strength is also good. The next day I went down to our local tractor supply type vendor and bought a 6' wide axle set. Tubing, wood, windows, rv battery, etc wound up costing about $4000. So yes you can build for a lot less than the $20 grand or so a factory one costs. One thing to consider is what I call mass centralization which means putting heavy items like an RV battery over the axle. It makes it easier to get the balance of the trailer set. Just my 2 cents worth.