I did this. Well, sort of.
Way back when I had to build a car trailer because I was racing cars. I didn't have any money but I could weld so I designed and built a flatbed car trailer. Used it for many years and it towed better than any trailer I have towed since.
I was always worried about someone taking parts off of my car when parked so I wanted an enclosed trailer. I still didn't have any money so I designed an enclosure that I welded up and put on my flatbed.
I used 4x8' sheet aluminum on the sides. I did not want any leaks in the roof so I went to the Fruehauf trailers and bought the same roll roofing they put on diesel tractor trailers. Thick stuff, but it goes from the very bottom in front to the very back of the trailer. One piece, no leaks. I didn't even drill holes in the roof to attach the roof because I didn't want anywhere for leaks to start at all. (Can you tell I hate leaks?)
Anyway, fast forward. I always wanted to stay at the track but had nowhere to sleep except the truck. I also didn't have anywhere to make food. So I designed a front area with a kitchen and stove and sink and microwave and what have you. I nabbed the stuff out of old campers and trailers over the years that my friends were going to throw away.
All of the front area is welded in place but none of the appliances are there yet. I was hopping to do that this summer.......but...
Anyway it looks really good and I'm very happy with my "toy hauler" that I made.
Now the bad part:
Would I recommend anybody do this? Can't say I can. I only saved a little money over a built trailer. The trailer manufactures buy their stuff by the train load. I buy one piece and they charge an arm and a leg for it so you don't really save that much. Don't get me wrong, I love my trailer but it was a LOT of work and time and money. That was when I was young and work was no big deal. Now days I'm lazy and don't want to spend 10 hrs a day welding up tubing and riveting aluminum paneling all day.
If you're looking for something to do; have at it. If you're trying to save money, I can't recommend doing this.
Good luck!
One other thing: You better really know what you're doing with weights and balances and where everything is going to go. If not, you can get in BIG trouble.