Ours is a TSC 4 x 8 1500lb trailer. It is made from 2 x 2 x 1/8" angle.
I'm hesitant to just add a receiver to the cross members. Seems kind of flimsy. The rear and the second cross members are about 20" apart. Then the axle is the next obstacle.
Adding more structural members is just going to add weight.
I'm now thinking of putting a piece of tube steel on top of the bare frame right down the center. This can be up to 8' long. I would then have to build a 2 x frame or at least some runners for the floor to raise the carcass.
This should be the cheapest and strongest way for this style of utility trailer.