Their exists a tool calles a shrinker that does what you want.
It is like two hardened steel jaws that grab and pull the metal thereby creating a rounded corner in a folded piece of flat sheet.
You'd basically make the major fold over and then using the shrinker pull the metal to create the rounded corners in the folded metal.
I had one and the learning curve was easy.
Worked best on aluminum and brass but OK on steel.