It would probably work just as well to measure and duplicate a top of the line factory built trailer. The ramps and hinges seem to be the hardest to fabricate. Resist the urge to use house trailer axles. Make the tongue adjustable at the trailer side. In my state (maybe nationwide) you must have a full deck. Northern sells tiedown rings as many trailer places do, I think they sell a book of trailer blueprints. It may be cheaper to purchase a trailer at auction and customize it to your needs. D.O.T. likes to see a weight placard, insurance co. like something they can place a value on easily.
For a 12k trailer, use 6 inch channel iron for the main frame rails, run 3” channel for cross member 24” on center is often enough although some trailers are built on 16” centers.
If you want a wood deck set your crossmembers down 2 inches from the top,
I prefer a short tongue so the trailer tracks closer to the truck on corners, say 40“ and not to much tongue weight, axels shoud be centered @ 60% of the deck length from the front unless your towing with a 1 ton or larger, then a 70/30 split is common. The latter may be prefered in mountainess country or if you like to load you machine near the back and have a couple attachments loaded from each side at the front
This should be stout enough for decks up to 20' long. 2 by 6 box iron makes a good feame to but tends to rust badly in our area and will be junk in 15 to 20 years.
If rust is a issue in your area stay with channel and avoid laying 1 piece of iron on top of another, and makeing area that can trap dirt.
Be sure to use rubber grommet mount lites from the trucking industy and soldier all conections including a ground back to the trucks plug in. Get yourself a 10k lb drop leg jack, it will last forever. I like to get my fenders made up from 1/8 sheet steel and get it bent in a brake, they don't have the nice curves, but are stout enough to stand our rough roads. Also I make them removeable, because fenders tend to get in the way at loading time for certain loads.