I designed and built my own tilt deck car hauler in 2017.
Took about 3 weeks, cost about 1/4 of what it would have cost to just buy a similar one, and weighs about 2/3 of what the store bought models weigh.
I did this both because I wanted a cheaper, better, lighter trailer, and because I had a really specific design requirement, in that I needed it to fit in here:
And there was just no way to make that happen with a store bought one.
It has held up extremely well and has seen extensive use for the last 4 years.
As far as design tips... I just overbuilt it. For figuring out how strong stuff needed to be, I looked at similar trailers and copied their design, or did it in ways that I thought would be stronger, or obviously overkill.
I really wanted low fenders AND a low deck, which is hard to do, so I built my fenders as close as I thought I could get away with to the tires, and made them so that if I had to make them taller it would be easy to modify. Luckily they work perfectly, I don't think I could have gone any lower without the tires rubbing, and they're low enough to open the doors on a corvette or skyline without them rubbing on the fender, which was what I was hoping for.
It's easy to make a much better trailer than the commercial ones, though, they're all designed to be cheap and fast to build, with very little effort put into making sure they are good at the job they are doing.