I agree with all the statements above. The design is poor, the material looks too think at the crack, etc.
What about cutting in three places and replacing the whole joint with a new material? Cut above the joint weld, and beyond the butt weld on the right, and far enough past the bend on the left. Fab a new section and have it welded in.
It's probably a LOT more work than patching or filling, but would probably be stronger in the long run.
What does the mfg. say? Will they help pay for the repair?