Cut two pieces of 3-by-4-inch angle iron 10 feet long and four 5-feet-long pieces with a metal chop saw. Miter the ends of the long pieces and two shorter pieces at 45-degree angles. Weld the two 10-foot pieces and two of the 5-foot pieces into a rectangular frame, with the 4-inch angle down and the 3-inch angle up on the sides and what will be the front. Weld the back with the 4-inch angle flat. Use a square to make sure corners are square.
Turn the frame over and measure the distance of the leaf spring mounts on the axle assembly. Be precise so holes in hangers and springs will match; position the mounts so 60 percent of the trailer will be in front of the axle. Weld the leaf spring hangers to the frame at those locations. Weld the other 5-foot sections as braces across the frame just in front of and just behind the leaf spring hangers. Put the 4-inch angle flat with the 3-inch angle facing down.
Set the frame on the axle assembly; brace it level with boards or blocks and fasten the springs to the hangers with bolts, lock washers and nuts.
Make a yoke for the trailer hitch. Cut two pieces of angle iron to go diagonally from the frame just in front of the front brace to the hitch cap; the angle and length will be determined by the angle on the hitch cap. Weld that yoke into place; add a cross brace of angle iron a foot or so behind the hitch ball cap.
Get 2-by-12-inch floor boards to fit; five boards 10 feet long should floor the trailer exactly. Fasten those boards with bolts or sheet metal screws; drill through the boards and the frame front, back and cross braces to secure the floorboards.