if you plan on hauling a rig on it, i doubt the angle will hold up as the side rails.
if you had another piece of the square tubing, thos would work really good for the sides, then use the angle as the cross beams, then all youll need is material for the tounge.
im in the process of gathering material for my trailer build. i scored a set of 3,500 lb brakeing axles for cheap. i only paid 150 for the axles. less then 500 miles from what the guy said.