I had to make a stainless steel rear rack because I did not build one onto the frame when I was building it.
Rear disc brake drop outs can be brazed in after you’re finished. Then build a rear rack right onto to the frame, making sure it clears the disc brake.
I have had way too many aluminum racks brake. Use 1/2 to 3/8th inch thick wall (.083" or .095") tubing of 4130 air plane steel. I use square tubes when possible because round tubes are harder to join to round tubes. But the round tubes do bend easier. Thin all tubing may have problems bending.
Flat bar works fine for the front, but angle bars may work better. I have seen one made of 3/4 inch flat bar But I used 1.5 inch by 1/8th inch on mine. But soon discovered that it was not wide enough to keep the box from wobbling side to side.