Its not hard to make a Trailer.
I hope you have a While to read.
I made a Tandem/Car Carrier from a Stuffed Caravan, Just removed the Caravan Shell & Cut off the Mounting Points leaving the Bare Chassis.
Then got a Sheet of Tread/Checker Plate, Cut it to approx size in the Guilitine @ Dad's(RIP) Work.(Can be done with a Grinder or Oxy) Then made New Brackets & welded them to the chassis for the Plate to weld too. I made the Sides out of 50mm X 20mm RHS Blue Tube. Cut the Uprights x 4 @ the Height I wanted with 22.5 degree Angle on the Base to be welded to the chassis, 22.5 Degree Angle on the Other end to be welded to the Length runners to be cut with the same angle to Make a /-------\ (Sorry about the Dodgy Pic, You get the Idea )
The Leaf Springs, I got a Extra 2 Leaves Added.
Brakes. As the Caravan was a Big Unit, It looked like it had Factory Ford F Series Drums on all 4's.
I hope that gives you some inspiration.